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Baudot code
Baudot code
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An early "piano" Baudot keyboard

The Baudot code (French pronunciation: [bodo]) is an early character encoding for telegraphy invented by Émile Baudot in the 1870s.[1] It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use before ASCII. Each character in the alphabet is represented by a series of five bits, sent over a communication channel such as a telegraph wire or a radio signal by asynchronous serial communication. The symbol rate measurement is known as baud, and is derived from the same name.

History

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Baudot code (ITA1)

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Baudot code (ITA1)
An early version from Baudot's 1888 US patent, listing A through Z, t and ∗ (Erasure)
Alias(es)International Telegraph Alphabet 1
Current statusReplaced by ITA2 (not mutually compatible).
Classification5-bit stateful[citation needed] basic Latin encoding
Preceded byMorse code
Succeeded byITA2

In the below table, Columns I, II, III, IV, and V show the code; the Let. and Fig. columns show the letters and numbers for the Continental and UK versions; and the sort keys present the table in the order: alphabetical, Gray and UK

Baudot code (Continental and UK versions).[2]
Europe sort keys UK sort keys
I II III IV V Con­ti­nen­tal Gray Let. Fig. I II III IV V UK
- - -
A 1 ● A 1 ●
É & ● ● / 1/ ● ●
E 2 ● E 2 ●
I o ● ● I 3/ ● ●
O 5 ● ● ● O 5 ● ● ●
U 4 ● ● U 4 ● ●
Y 3 ● Y 3 ●
B 8 ● ● B 8 ● ●
C 9 ● ● ● C 9 ● ● ●
D 0 ● ● ● ● D 0 ● ● ● ●
F f ● ● ● F 5/ ● ● ●
G 7 ● ● G 7 ● ●
H h ● ● ● H ¹ ● ● ●
J 6 ● ● J 6 ● ●
Figure Blank ● Fig. Bl. ●
Erasure Erasure ● ● * * ● ●
K ( ● ● ● K ( ● ● ●
L = ● ● ● ● L = ● ● ● ●
M ) ● ● ● M ) ● ● ●
N N° ● ● ● ● N £ ● ● ● ●
P % ● ● ● ● ● P + ● ● ● ● ●
Q / ● ● ● ● Q / ● ● ● ●
R – ● ● ● R – ● ● ●
S ; ● ● S 7/ ● ●
T ! ● ● ● T ² ● ● ●
V ' ● ● ● ● V ¹ ● ● ● ●
W ? ● ● ● W ? ● ● ●
X , ● ● X 9/ ● ●
Z : ● ● ● Z : ● ● ●
t . ● ● – . ● ●
Blank Letter ● Bl. Let. ●

Baudot developed his first multiplexed telegraph in 1872[3][4] and patented it in 1874.[4][5] In 1876, he changed from a six-bit code to a five-bit code,[4] as suggested by Carl Friedrich Gauss and Wilhelm Weber in 1834,[3][6] with equal on and off intervals, which allowed for transmission of the Roman alphabet, and included punctuation and control signals. The code itself was not patented (only the machine) because French patent law does not allow concepts to be patented.[7]

Baudot's 5-bit code was adapted to be sent from a manual keyboard, and no teleprinter equipment was ever constructed that used it in its original form.[8] The code was entered on a keyboard which had just five piano-type keys and was operated using two fingers of the left hand and three fingers of the right hand. Once the keys had been pressed, they were locked down until mechanical contacts in a distributor unit passed over the sector connected to that particular keyboard, at which time the keyboard was unlocked ready for the next character to be entered, with an audible click (known as the "cadence signal") to warn the operator. Operators had to maintain a steady rhythm, and the usual speed of operation was 30 words per minute.[9]

The table "shows the allocation of the Baudot code which was employed in the British Post Office for continental and inland services. A number of characters in the continental code are replaced by fractionals in the inland code. Code elements 1, 2 and 3 are transmitted by keys 1, 2 and 3, and these are operated by the first three fingers of the right hand. Code elements 4 and 5 are transmitted by keys 4 and 5, and these are operated by the first two fingers of the left hand."[8][10][11]

Baudot's code became known as the International Telegraph Alphabet No. 1 (ITA1). It is no longer used.

Murray code

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Paper tape with holes representing the "Baudot–Murray Code". Note the fully punched columns of "Delete/Letters select" codes at end of the message (on the right) which were used to cut the band easily between distinct messages. The last symbols before the fully punched columns at the end are BRASIL CR LF CR FS (word Brasil, carriage return, line feed, carriage return, shift to figures)

In 1901, Baudot's code was modified by Donald Murray (1865–1945), prompted by his development of a typewriter-like keyboard. The Murray system employed an intermediate step: an operator used a keyboard perforator to punch a paper tape and then a transmitter to send the message from the punched tape. At the receiving end of the line, a printing mechanism would print on a paper tape, and/or a reperforator would make a perforated copy of the message.[12]

Because there was no longer a connection between the operator's hand movement and the bits transmitted, there was no concern about arranging the code to minimize operator fatigue. Instead, Murray designed the code to minimize wear on the machinery by assigning the code combinations with the fewest punched holes to the most frequently used characters.[13][14] For example, the one-hole letters are E and T. The ten two-hole letters are AOINSHRDLZ, very similar to the "Etaoin shrdlu" order used in Linotype machines. Ten more letters, BCGFJMPUWY, have three holes each, and the four-hole letters are VXKQ.

The Murray code also introduced what became known as "format affectors" or "control characters" – the CR (Carriage Return) and LF (Line Feed) codes. A few of Baudot's codes moved to the positions where they have stayed ever since: the NULL or BLANK and the DEL code. NULL/BLANK was used as an idle code for when no messages were being sent, but the same code was used to encode the space separation between words. Sequences of DEL codes (fully punched columns) were used at start or end of messages or between them which made it easier to separate distinct messages. (BELL codes could be inserted in those sequences to signal to the remote operator that a new message was coming or that transmission of a message was terminated).

Early British Creed machines also used the Murray system.

Western Union

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Keyboard of a teleprinter using the Baudot code (US variant), with FIGS and LTRS shift keys

Murray's code was adopted by Western Union which used it until the 1950s, with a few changes that consisted of omitting some characters and adding more control codes. An explicit SPC (space) character was introduced, in place of the BLANK/NULL, and a new BEL code rang a bell or otherwise produced an audible signal at the receiver. Additionally, the WRU or "Who aRe yoU?" code was introduced, which caused a receiving machine to send an identification stream back to the sender.

ITA2

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ITA2 Baudot–Murray code
British variant of ITA2
Alias(es)International Telegraph Alphabet 2
Classification5-bit stateful[citation needed] basic Latin encoding
Preceded byITA1
Succeeded byFIELDATA,
ITA 3 (van Duuren code),
ITA 5 (ISO 646, ASCII)
MTK-2
LanguageRussian
Classification5-bit stateful[citation needed] Russian Cyrillic encoding
Preceded byRussian Morse code
Succeeded byKOI-7

In 1932, the CCITT introduced the International Telegraph Alphabet No. 2 (ITA2) code[15] as an international standard, which was based on the Western Union code with some minor changes. The US standardized on a version of ITA2 called the American Teletypewriter code (US TTY) which was the basis for 5-bit teletypewriter codes until the debut of 7-bit ASCII in 1963.[16]

Some code points (marked blue in the table) were reserved for national-specific usage.[17]

A four-row teletype keyboard with Roman and Cyrillic letters.
International telegraphy alphabet No. 2 (Baudot–Murray code)[18]
Impulse patterns
(1=mark, 0=space) 		Letter shift Figure shift
LSB on
right;
code elements:
543·21 		LSB on
left;
code elements:
12·345 		Count of punched marks ITA2
standard 		Russian
MTK-2
variant 		Russian
MTK-2
variant 		ITA2
standard 		US TTY
variant
000·00 00·000 0 Null Shift to Russian Letters (RS) Null
010·00 00·010 1 Carriage return
000·10 01·000 1 Line feed
001·00 00·100 1 Space
101·11 11·101 4 Q Я 1
100·11 11·001 3 W В 2
000·01 10·000 1 E Е 3
010·10 01·010 2 R Р 4, Ч 4
100·00 00·001 1 T Т 5
101·01 10·101 3 Y Ы 6
001·11 11·100 3 U У 7
001·10 01·100 2 I И 8
110·00 00·011 2 O О 9
101·10 01·101 3 P П 0
000·11 11·000 2 A А –
001·01 10·100 2 S С ' Bell
010·01 10·010 2 D Д WRU? $
011·01 10·110 3 F Ф Э !
110·10 01·011 3 G Г Ш &
101·00 00·101 2 H Х Щ £ #
010·11 11·010 3 J Й Ю, Bell Bell '
011·11 11·110 4 K К (
100·10 01·001 2 L Л )
100·01 10·001 2 Z З + "
111·01 10·111 4 X Ь /
011·10 01·110 3 C Ц :
111·10 01·111 4 V Ж = ;
110·01 10·011 3 B Б ?
011·00 00·110 2 N Н ,
111·00 00·111 3 M М .
110·11 11·011 4 Shift to Figures (FS) Reserved for
figures extension
111·11 11·111 5 Reserved for
lettercase extension		 Shift to Letters (LS)
/ Erasure / Delete

The code position assigned to Null was in fact used only for the idle state of teleprinters. During long periods of idle time, the impulse rate was not synchronized between both devices (which could even be powered off or not permanently interconnected on commuted phone lines). To start a message it was first necessary to calibrate the impulse rate, a sequence of regularly timed "mark" pulses (1), by a group of five pulses, which could also be detected by simple passive electronic devices to turn on the teleprinter. This sequence of pulses generated a series of Erasure/Delete characters while also initializing the state of the receiver to the Letters shift mode. However, the first pulse could be lost, so this power on procedure could then be terminated by a single Null immediately followed by an Erasure/Delete character. To preserve the synchronization between devices, the Null code could not be used arbitrarily in the middle of messages (this was an improvement to the initial Baudot system where spaces were not explicitly differentiated, so it was difficult to maintain the pulse counters for repeating spaces on teleprinters). But it was then possible to resynchronize devices at any time by sending a Null in the middle of a message (immediately followed by an Erasure/Delete/LS control if followed by a letter, or by a FS control if followed by a figure). Sending Null controls also did not cause the paper band to advance to the next row (as nothing was punched), so this saved precious lengths of punchable paper band. On the other hand, the Erasure/Delete/LS control code was always punched and always shifted to the (initial) letters mode. According to some sources, the Null code point was reserved for country-internal usage only.[17]

The Shift to Letters code (LS) is also usable as a way to cancel/delete text from a punched tape after it has been read, allowing the safe destruction of a message before discarding the punched band.[clarification needed] Functionally, it can also play the same filler role as the Delete code in ASCII (or other 7-bit and 8-bit encodings, including EBCDIC for punched cards). After codes in a fragment of text have been replaced by an arbitrary number of LS codes, what follows is still preserved and decodable. It can also be used as an initiator to make sure that the decoding of the first code will not give a digit or another symbol from the figures page (because the Null code can be arbitrarily inserted near the end or beginning of a punch band, and has to be ignored, whereas the Space code is significant in text).

The cells marked as reserved for extensions (which use the LS code again a second time—just after the first LS code—to shift from the figures page to the letters shift page) has been defined to shift into a new mode. In this new mode, the letters page contains only lowercase letters, but retains access to a third code page for uppercase letters, either by encoding for a single letter (by sending LS before that letter), or locking (with FS+LS) for an unlimited number of capital letters or digits before then unlocking (with a single LS) to return to lowercase mode.[19] The cell marked as "Reserved" is also usable (using the FS code from the figures shift page) to switch the page of figures (which normally contains digits and national lowercase letters or symbols) to a fourth page (where national letters are uppercase and other symbols may be encoded).

ITA2 is still used in telecommunications devices for the deaf (TDD), Telex, and some amateur radio applications, such as radioteletype ("RTTY"). ITA2 is also used in Enhanced Broadcast Solution, an early 21st-century financial protocol specified by Deutsche Börse, to reduce the character encoding footprint.[20]

Nomenclature

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Nearly all 20th-century teleprinter equipment used Western Union's code, ITA2, or variants thereof. Radio amateurs casually call ITA2 and variants "Baudot" incorrectly,[21] and even the American Radio Relay League's Amateur Radio Handbook does so, though in more recent editions the tables of codes correctly identifies it as ITA2.

Character set

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The values shown in each cell are the Unicode codepoints, given for comparison.

Original Baudot variants

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Original Baudot, domestic UK

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Original Baudot code, UK domestic variant (letter set, switched to with 0x10)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL A E / Y U I O FIGS J G H B C F D
1x  SP  - X Z S T W V DEL K M L R Q N P
Original Baudot code, UK domestic variant (figure set, switched to with 0x08)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL 1 2 ⅟ 3 4 ³⁄ 5  SP  6 7 ¹ 8 9 ⁵⁄ 0
1x LTRS . ⁹⁄ : ⁷⁄ ² ? ' DEL ( ) = - / £ +

Original Baudot, Continental European

[edit]
Original Baudot code, continental European variant (letter set, switched to with 0x10)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL A E É Y U I O FIGS J G H B C F D
1x  SP  ṯ X Z S T W V DEL K M L R Q N P
Original Baudot code, continental variant (figure set, switched to with 0x08)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL 1 2 & 3 4 º 5  SP  6 7 ʰ̵ 8 9 ᶠ̵ 0
1x LTRS . , : ; ! ? ' DEL ( ) = - / № %

Original Baudot, ITA 1

[edit]
ITA 1 (letter set, switched to with 0x10)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL A E  CR  Y U I O FIGS J G H B C F D
1x  SP   LF  X Z S T W V DEL K M L R Q N P
ITA 1 (figure set, switched to with 0x08)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL 1 2  CR  3 4 PU[a] 5  SP  6 7 + 8 9 PU[a] 0
1x LTRS  LF  , : . PU[a] ? ' DEL ( ) = - / PU[a] %

Baudot–Murray variants

[edit]

Murray Code

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Murray code (letter set, switched to with 0x04)[22]
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x  SP  E COL A LTRS S I U  LF  D R J N F C K
1x T Z L W H Y P Q O B G FIGS M X V DEL/*[b]
Murray code (figure set, switched to with 0x1B)
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x  SP  3 COL LTRS ' 8 7  LF  ² 4 ⁷⁄ − ⅟ ( ⁹⁄
1x 5 . / 2 ⁵⁄ 6 0 1 9 ? ³⁄ FIGS , £ ) DEL/*[b]

ITA 2 and US-TTY

[edit]
ITA2 and US-TTY Baudot–Murray code (letter set, switched to with 0x1F)
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL E  LF  A  SP  S I U  CR  D R J N F C K
1x T Z L W H Y P Q O B G FIGS M X V LTRS/DEL
US-TTY Baudot–Murray code (figure set, switched to with 0x1B)
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL 3  LF  −  SP  BEL 8 7  CR  $ 4 ' , ! : (
1x 5 " ) 2 # 6 0 1 9 ? & FIGS . / ; LTRS
ITA2 Baudot–Murray code (figure set, switched to with 0x1B)
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x NUL 3  LF  −  SP  ' 8 7  CR  ENQ 4 BEL , ! : (
1x 5 + ) 2 £ 6 0 1 9 ? & FIGS . / = LTRS

Weather code

[edit]

Meteorologists used a variant of ITA2 with the figures-case symbols, except for the ten digits, BEL and a few other characters, replaced by weather symbols:

Weather teleprinter encoding
Meteorological Baudot–Murray code (figure set, switched to with 0x1B)
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x - 3  LF  ↑  SP  BEL 8 7  CR  ↗ 4 ↙ ⦷ → ◯ ←
1x 5 + ↖ 2 ↓ 6 0 1 9 ⊕ ↘ FIGS . / ⦶ LTRS

Details

[edit]

Note: This table presumes the space called "1" by Baudot and Murray is rightmost, and least significant. The way the transmitted bits were packed into larger codes varied by manufacturer. The most common solution allocates the bits from the least significant bit towards the most significant bit (leaving the three most significant bits of a byte unused).

Table of ITA2 codes (expressed as hexadecimal numbers)

In ITA2, characters are expressed using five bits. ITA2 uses two code sub-sets, the "letter shift" (LTRS), and the "figure shift" (FIGS). The FIGS character (11011) signals that the following characters are to be interpreted as being in the FIGS set, until this is reset by the LTRS (11111) character.[23] In use, the LTRS or FIGS shift key is pressed and released, transmitting the corresponding shift character to the other machine. The desired letters or figures characters are then typed. Unlike a typewriter or modern computer keyboard, the shift key isn't kept depressed whilst the corresponding characters are typed. "ENQuiry" will trigger the other machine's answerback. It means "Who are you?"

CR is carriage return, LF is line feed, BEL is the bell character which rang a small bell (often used to alert operators to an incoming message), SP is space, and NUL is the null character (blank tape).

Note: the binary conversions of the codepoints are often shown in reverse order, depending on (presumably) from which side one views the paper tape. Note further that the "control" characters were chosen so that they were either symmetric or in useful pairs so that inserting a tape "upside down" did not result in problems for the equipment and the resulting printout could be deciphered. Thus FIGS (11011), LTRS (11111) and space (00100) are invariant, while CR (00010) and LF (01000), generally used as a pair, are treated the same regardless of order by page printers.[24] LTRS could also be used to overpunch characters to be deleted on a paper tape (much like DEL in 7-bit ASCII).

The sequence RYRYRY... is often used in test messages, and at the start of every transmission. Since R is 01010 and Y is 10101, the sequence exercises much of a teleprinter's mechanical components at maximum stress. Also, at one time, fine-tuning of the receiver was done using two coloured lights (one for each tone). 'RYRYRY...' produced 0101010101..., which made the lights glow with equal brightness when the tuning was correct. This tuning sequence is only useful when ITA2 is used with two-tone FSK modulation, such as is commonly seen in radioteletype (RTTY) usage.

US implementations of Baudot code may differ in the addition of a few characters, such as #, & on the FIGS layer.

The Russian version of Baudot code (MTK-2) used three shift modes; the Cyrillic letter mode was activated by the character (00000). Because of the larger number of characters in the Cyrillic alphabet, the characters !, &, £ were omitted and replaced by Cyrillics, and BEL has the same code as Cyrillic letter Ю. The Cyrillic letters Ъ and Ё are omitted, and Ч is merged with the numeral 4.

See also

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Explanatory notes

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Baudot code

View on Grokipedia
from Grokipedia
Baudot code is a pioneering 5-bit binary system developed by French engineer in 1870 for use in , enabling the transmission of letters, numbers, and symbols over electrical lines using fixed-length codewords. This code represented one of the earliest practical digital communication standards, utilizing 32 unique combinations to encode characters while employing shift mechanisms—such as "letters" (LTRS) and "figures" (FIGS) modes—to access a broader set of symbols beyond the basic 32, thereby overcoming the limitations of purely alphabetic or numeric systems. Baudot's innovation stemmed from his work as a telegraph operator starting in , leading to a patented system that allowed multiple receivers on a single wire through via a distributor mechanism. Unlike the variable-length dots and dashes of , which had been in use since the , Baudot code's fixed 5-bit structure (often transmitted at 45 bits per second with start and stop bits in asynchronous modes) significantly increased transmission speeds and efficiency for . Operators interacted with it via a five-key musical-style keyboard that punched codes onto paper tape, which could then be read mechanically for automated sending and receiving. The system gained international adoption after Baudot's patents in France, England, Germany, and the United States (U.S. Patent No. 388,244, granted August 21, 1888), with prototypes tested in the late 1870s and widespread deployment in France by 1892. Variants emerged, including the original French version and the Anglo-American Baudot-Murray code, which influenced the International Telegraph Alphabet No. 2 (ITA2) standardized by the International Telegraph Consultative Committee in 1930 and used for over four decades in teleprinters. North American adaptations like USTTY introduced minor modifications for regional needs. Baudot code's legacy extends to modern computing, serving as a precursor to ASCII and other character encodings by establishing binary principles for representation. It was employed in early electronic computers during the and , such as for punch-card systems, and its impact is commemorated in the unit of signaling speed, the "baud," named after its inventor. By the mid-20th century, it had largely supplanted in printing telegraphy, marking a foundational shift toward digital telecommunications.

History

Invention and Early Development

Émile Baudot, a French engineer born in 1845 in Magneux, , joined the French postal and telegraph administration in 1870 after working on his family's farm. That same year, while employed at the Central Post Office in , he began developing a system that utilized a 5-bit to enable direct transmission and printing of text on paper strips at the receiving end. This innovation marked an early step toward digital communication, predating later codes like ASCII by encoding letters, numbers, and symbols into uniform binary sequences. In 1874, Baudot filed a patent for his "System for Fast Telegraphy" (French Patent No. 103,898), describing a synchronous multiplex telegraph machine that employed 5-bit binary encoding to represent 32 symbols. The device featured a 5-key keyboard operated by the fingers of one hand, requiring operators to maintain a steady rhythm at speeds up to 30 words per minute, with signals transmitted in equal on-and-off intervals over shared lines using time-division multiplexing to allow multiple simultaneous transmissions. This system represented the first practical application of such multiplexing in telegraphy, enabling efficient use of a single wire for several messages. Key milestones followed the patent, including successful tests of the system on November 12, 1877, between and , and a longer 1,700 km link from to by year's end. In 1878, Baudot demonstrated his multiplex at the Paris Universal Exposition, earning a and gaining international recognition for the technology. The French postal service adopted the system in 1880, marking its first widespread real-world deployment for transmitting messages over shared lines. Early implementations faced challenges, particularly the synchronous operation, which demanded precise clocking via an audible "cadence signal" to synchronize sender and receiver, and the limitation to 32 characters due to the 5-bit structure without additional mechanisms. Despite these hurdles, the Baudot code's design laid foundational principles for binary telegraphy and influenced subsequent communication systems.

Standardization and Key Variants

The standardization of the Baudot code evolved significantly in the early through international efforts to unify telegraph alphabets for global compatibility. The first major step toward occurred at the 1908 International Telegraph Conference in , where regulations adopted an early form of the Baudot-based alphabet, laying the groundwork for what became the International Telegraph Alphabet No. 1 (ITA1). This was further refined during subsequent meetings of the International Consultative Committee for Telegraphs and Telephones (CCIT, predecessor to the CCITT), culminating in the 1929 where ITA1 was formally proposed as a standardized five-unit code derived from Baudot's original system. Adoption of ITA1 followed in 1932 at the Madrid International Telecommunication Conference, marking its official endorsement for international and emphasizing synchronous transmission for multiplex systems. Parallel developments focused on asynchronous variants to enhance compatibility with non-synchronized equipment, particularly for tape-based printing telegraphs. In the early , engineers adapted the Baudot code to include start-stop signaling, allowing independent operation without precise , which broadened its use in diverse telegraph networks. A pivotal advancement came from Donald Murray, who in began improving the code after moving to the from ; his innovations included rearranging character assignments for frequency optimization, introducing shift mechanisms for case switching, and incorporating basic error detection via parity-like checks in transmission. Murray patented these enhancements starting in 1902 (US Patent 710,163, issued September 30, 1902, for code improvements), followed by a series of filings through 1914 that refined punched-tape perforators and automatic actuation for reliable asynchronous operation. In the United States, played a key role in domestic adoption and modification during the and . The company acquired Murray's patents in and integrated a hybrid Murray-Morkrum code variant into its multiplex systems by , tailoring it for high-volume traffic with adjustments to support page printing and reduced errors in long-distance lines. By the , had expanded these modifications across its network, incorporating asynchronous start-stop protocols to interface with emerging teletypewriters, which facilitated faster message handling in commercial . The culmination of these efforts arrived with the standardization of the International Telegraph Alphabet No. 2 (ITA2) by the CCITT in 1930, building directly on Murray's five-bit code augmented by two shift bits to encode up to 72 characters, including letters, figures, and controls. This standard, ratified at the 1932 , superseded ITA1 for most applications by prioritizing efficiency in asynchronous teleprinters while maintaining with Baudot's foundational principles. A timeline of key patents underscores this progression: Baudot's original 1874 patent (FR 103,898) for the multiplex system; Murray's 1902-1914 series (e.g., US 710,163 in 1902, US 1,097,981 in 1914 for error-handling mechanisms); and the international accords from (1908) through (1932) that formalized global variants.

Nomenclature

Naming Origins

The Baudot code derives its name from the French telegraph engineer Jean-Maurice-Émile Baudot, who developed the original five-unit encoding system and received a for it on June 17, 1874 (French patent no. 103,898). This attribution honors Baudot's pioneering role in creating a fixed-length binary telegraph , even though later adaptations significantly altered the original design by incorporating shift mechanisms and expanded character sets. Common alternative names for the code and its variants include the International Teleprinter Code, reflecting its widespread adoption in systems, and simply "" in early documentation. A frequent arises with the term "Baudot-Murray code," which typically denotes post-1900 modifications introduced by Australian engineer Donald Murray, who refined the system for practical printing telegraphs; however, the pure Baudot code strictly refers to the 1874 original without these enhancements. Murray himself contributed to the confusion by broadly labeling any five-unit code as the "Baudot alphabet" in his writings and patents. The nomenclature evolved from "code Baudot" or "Baudot's code" in 19th-century French technical documents to more formalized international designations, such as CCITT No. 2 (now ) by the 1930s, following the 1929 standardization of variants as International Telegraph Alphabet No. 1 (ITA1) and No. 2 (ITA2). Today, "Baudot code" commonly denotes ITA2 in and contexts, rather than the unmodified 1874 version.

Telegraph Alphabet Standards

The International Telegraph Alphabets (ITAs) were developed under the auspices of the International Telegraph Union (ITU), established in 1865 as the precursor to the modern International Telecommunication Union, to standardize character encoding for telegraphy and ensure compatibility across international networks, particularly for mechanical printing telegraphs. These standards aimed to replace variable-length codes like Morse with fixed-length binary sequences, enabling automated transmission and reception at higher speeds. International Telegraph Alphabet No. 1 (ITA1), formalized in 1929 by the International Consultative Committee for Telegraphs (CCIT) at its meeting, provided a 5-bit supporting a basic 32-character set without dedicated case-shifting mechanisms, though it incorporated letter and figure space shifts for accessing numerals and symbols; it was primarily adopted for European telegraph systems to promote uniformity in continental operations. This standard marked a shift from Morse's irregular pulse patterns to consistent unit durations, simplifying mechanical decoding and reducing errors in printing telegraphs. International Telegraph Alphabet No. 2 (ITA2), standardized in 1930–1931 by the CCIT following deliberations at conferences including , introduced a refined 5-bit code with explicit letters (LTRS) and figures (FIGS) shift controls, enabling access to up to 72 symbols including letters, numerals, and punctuation for broader international compatibility. Adopted globally through CCIT regulations, ITA2 became the dominant standard for services like , supporting mechanical printers and radio teleprinters worldwide. Subsequent revisions to ITA2 occurred through the 1960s, including updates at the (1960) and (1964) conferences by the CCITT (successor to CCIT, formed in at the International Telegraph Conference), which incorporated provisions for error detection such as dedicated error characters and optional extensions for national variants. These enhancements, detailed in CCITT Recommendations C.7, C.8, and C.12, addressed reliability in long-distance transmissions without altering the core 5-bit structure. The 1927 Washington International Radiotelegraph Conference and 1932 Madrid Radiotelegraph Conference laid groundwork for these evolutions by harmonizing radio and wire telegraph standards under ITU oversight.

Technical Principles

Encoding Structure

The Baudot code employs a 5-bit binary architecture, utilizing two signal polarities—mark (representing binary 1, often a negative voltage or active state) and (representing binary 0, often a positive voltage or idle state)—to encode up to 32 distinct combinations per character. This fixed-length format enabled efficient in early telegraph systems, where each bit duration corresponded to a in the signal stream. In the original 1874 design, transmission was synchronous, delivering a continuous bit stream at a fixed baud rate, such as 50 baud, without the need for start or stop bits to delineate characters; synchronization relied on a shared clock between sender and receiver. Later asynchronous adaptations, introduced post-1900 for compatibility with start-stop teleprinters, prefixed each 5-bit character with a start bit (space polarity) and suffixed it with 1.5 to 2 stop bits (mark polarity), extending the total frame to 7 to 8 bits per character to allow independent clock recovery at the receiver. Bits within each character are transmitted starting with the least significant bit first, facilitating mechanical and electromechanical processing in telegraphic equipment. The baud rate, a unit named after denoting signal changes per second, standardized at 45.45 baud for the International Telegraph Alphabet No. 2 (ITA2) variant to support practical transmission speeds equivalent to about 60 words per minute in applications. For error detection, Baudot systems incorporated idle channel monitoring, maintaining the line in the mark polarity during non-transmission periods to identify anomalies such as unexpected spaces or noise-induced transitions.

Shift and Control Mechanisms

The Baudot code, limited to 32 distinct 5-bit combinations, employs shift mechanisms to access a broader set of 60 or more symbols by toggling between two modes: letters (alphabetic characters) and figures (numeric and punctuation symbols). Two dedicated codes serve this function: the letters shift (LTRS, binary 11111) activates the alphabetic mode, while the figures shift (FIGS, binary 11011) activates the numeric/symbol mode. These shifts are interpreted by the receiving device to reinterpret subsequent character codes from the appropriate set until the opposite shift is received. The mode set by a shift persists across multiple characters, enabling efficient transmission of sequences in a single case without repeated shifting; for instance, a series of letters remains in LTRS mode until FIGS is explicitly sent. This latching behavior contrasts with momentary shifts in later codes and allows continuous typing in one mode, though it requires careful management to avoid mode mismatches between sender and receiver. To ensure , transmissions often begin with two LTRS codes, guaranteeing the receiver starts in alphabetic mode. Several of the 32 codes are allocated to control functions essential for formatting and operation, including (binary 00100), which advances the mechanism without printing; (CR, binary 01000), which returns the print head to the line start; and line feed (LF, binary 00010), which advances the paper to the next line. These controls operate independently of the current shift mode, providing reliable formatting regardless of whether letters or figures are active. An additional null or blank code (binary 00000) serves as an idle pattern, producing no output or movement and stabilizing transmission speed or motor control without altering content. Error recovery in Baudot systems relies on unshift operations and standardized end sequences to reset modes and correct minor discrepancies; the LTRS code not only shifts but can also "wipe" or ignore erroneous tape sections in perforated media without disrupting the overall message. Messages typically conclude with a sequence like CR LF LTRS LTRS, which positions the device properly and reverts to letters mode for the next transmission, mitigating desynchronization from noise or errors. Some implementations include machine-specific behaviors, such as automatic unshift after a or at line ends, to further aid recovery. A key limitation of these mechanisms is the absence of lowercase support; all alphabetic characters are uppercase only, with shifts exclusively handling the transition to numbers and for the 26 letters plus controls. For example, to transmit "E3", the sequence begins with LTRS followed by the for E (binary 00001 in letters mode, interpreted as the letter), then FIGS followed by the same (now as 3 in figures mode); if subsequent letters follow, another LTRS is required to revert the mode. This dual interpretation expands utility but demands precise shift insertion to prevent garbled output.

Character Sets

Original 1874 Baudot Code

The original 1874 Baudot code, patented by French engineer , represented a pioneering 5-bit binary encoding system for , enabling the transmission of characters using fixed-length units of five equal-duration pulses. This design was tailored for synchronous printing telegraphs, replacing the variable-length and allowing for automated printing at the receiving end. Optimized for the , the code prioritized frequent characters such as vowels () in efficient assignments to minimize transmission errors and time. It used basic shift mechanisms to access figures and symbols beyond the primary alphabetic set, while excluding most diacritics to fit within the 32-symbol capacity per mode. The set encompassed uppercase letters A–Z (including J and Q, with some regional omissions later), the space character, and in figures mode, digits 0–9 and basic including the period and . The encoding used 5-bit binary sequences, where each bit corresponded to a telegraph signal: a dot for positive voltage (often represented as or a in paper tape) and a or for negative voltage (1 or a mark). Early representations sometimes depicted these as dots and analogous to Morse, but the uniform pulse duration distinguished it as a true . equivalents ranged from to 31, with binary assignments chosen to balance frequency of use and reliability. Below is a table of representative bit assignments from the original code, illustrating the structure (binary written with the first transmitted bit on the left; historical notations sometimes reverse this).
CharacterBinaryDecimalTelegraph Signals (dot = 0, dash = 1)
Space000000·····
A1100024−−···
E1010020−· ···
I0110012··−−·
O0101010· − · − ·
. (period)001015·· ·−−
01101026−− ·− ·
This structure, with shifts for letters and figures modes, limited the code to 32 symbols per mode, precluding lowercase letters and extended characters, and required synchronous operation between sender and receiver without advanced error correction—making it suitable for high-speed text transmission in controlled telegraph networks but leading to variants for broader use.

Regional Original Variants

Following the invention of the 1874 Baudot code, regional variants emerged in the 1880s as national telegraph administrations modified the 5-bit structure to better support local alphabets and symbols, while preserving the 32-code limit and basic shift mechanisms. These adaptations optimized for language-specific frequency distributions and practical telegraph use, diverging from the French baseline in character mappings and bit assignments. In the , the domestic variant was tailored for English-language by the , with adoption beginning in the early for inland circuits. Adjustments included adding the £ symbol, remapping less frequent letters like J and , and reassigning bits for high-frequency characters such as E and T to reduce transmission errors. For instance, the character was encoded as 11000 in the UK variant, contrasting with 00000 in the original French code. The following table illustrates key divergences in bit assignments (transmission order left-to-right, 1=mark, 0=):
CharacterOriginal 1874 French (bits)UK Domestic Variant (bits)
Space0000011000
A1100010000
E1010000011
£N/A01110
J0011111100 (remapped)
These changes enhanced efficiency for English text while omitting French-specific accents like É. The variant was deployed on duplex circuits for domestic services, supporting the Post Office's growing inland network by the mid-1880s. Continental European variants, developed concurrently in the 1880s for postal telegraph networks, focused on accommodating accented characters and diacritics in languages like French and German. French implementations retained support for but refined mappings for smoother keyboard operation on 5-key distributors. German variants remapped codes for umlauts (, , ), often by modifying base letter assignments—e.g., as a variant of A (11000 base). Bit differences emphasized regional and numerals, such as reassigning 01010 from a minor symbol to in German use. Adoption spread across European postal systems in the 1880s, with leading on multiplex lines and integrating variants into its networks for national and cross-border traffic. Common to all variants was the retention of 32 fixed codes per mode for simplicity in mechanical printing telegraphs, prioritizing language optimizations over expanded repertoires.

International Telegraph Alphabet No. 1 (ITA1)

The International Telegraph Alphabet No. 1 (ITA1), standardized in the early 1900s following international telegraph conferences around 1900–1901, represented the first global adaptation of Émile Baudot's telegraph code for broader compatibility across major languages including English, French, and Russian. This 5-bit encoding scheme enabled transmission over synchronous telegraph systems, utilizing 32 possible combinations per mode to support essential characters for international messaging, while prioritizing efficiency in mechanical equipment. ITA1 marked a shift from purely regional variants by establishing a unified framework that facilitated in European and transatlantic telegraph networks, though limited to uppercase letters and basic symbols due to fixed-length binary signaling constraints. ITA1's character set consisted of a core 32-symbol repertoire per mode, encompassing the 26 uppercase letters A–Z, digits 0–9 (accessed via figures shift), a space character, and essential punctuation such as period (.), comma (,), colon (:), semicolon (;), question mark (?), apostrophe ('), hyphen (-), parentheses (()), and quotation marks ("), alongside control functions like carriage return and line feed. Omissions included lowercase letters and most accented characters (except by agreement), reflecting focus on concise telegraphic communication. This design balanced numerical data needs in commercial telegrams with alphabetic primacy, using letters (default) and figures modes to expand utility within the 5-bit limit. ITA1 employed a shift mechanism (letters shift for alphabetic mode, figures shift for numerals/), with the overall structure fixed in 5-unit format and no advanced dynamic changes, distinguishing it from ITA2. Bit assignments followed a standardized table, with each character a unique 5-bit sequence of mark (1) or space (0) pulses, optimized for synchronous operation. For instance, was 10000 (letters) / 3 (figures), ensuring reliable decoding through balanced bit patterns. The following table illustrates representative bit mappings from ITA1 assignments, showing letters and figures modes (binary left-to-right in transmission order, leftmost first unit):
Character (Letters Mode)BinaryCharacter (Figures Mode)Binary
A11000-11000
E10000310000
I01100801100
Space01000Space01000
. (period)11110. (period)11110
, (comma)01010, (comma)01010
These mappings derived from early 20th-century consensus, documented in CCITT regulations like the 1932 Telegraph Regulations for consistent implementation. ITA1's structure emphasized simplicity, assigning frequent letters like and A to balanced codes to minimize errors in electromagnetic systems.

Murray Code and ITA2

The Murray code, developed by Donald Murray between 1904 and 1914, represented a significant advancement in printing telegraphy by refining letter shift (LTRS) and figure shift (FIGS) mechanisms to expand the 32 possible 5-bit codes into a repertoire of up to 72 symbols. This shift system allowed operators to toggle between alphabetic characters and numerals/punctuation using dedicated control codes, enabling more efficient use of standard keyboards while minimizing mechanical wear. Murray's innovations built on earlier 5-unit codes by incorporating practical controls like and line feed, facilitating page-based printing in synchronous systems. In 1930, the International Telegraph Consultative Committee (CCITT, now ) officially adopted a refined version of the Murray code as International Telegraph Alphabet No. 2 (ITA2), standardizing 5-bit assignments for international use. ITA2 retained the LTRS/FIGS shift structure, with LTRS as 11111 and FIGS as 11011; for example, 00011 is 'A' in LTRS but unused in FIGS, while 11001 is 'B' in LTRS and '?' in FIGS. This ensured interoperability across global networks, prioritizing frequent English characters. ITA2's character set encompasses 26 letters, 10 digits, common , and controls, divided into LTRS (alphabetic) and FIGS (numeric/symbolic) modes. The full mappings are as follows (binary left-to-right, transmission order):
BinaryLTRSFIGSBinaryLTRSFIGSBinaryLTRSFIGS
00000NULNUL10100H$11100M.
00001E310101Y611101X/
00010LFLF10110P011110V(
00011A-10111Q111111LTRSLTRS
00100SPSP11000O911011FIGSFIGS
00101S'11001B?01000CRCR
00110I811010G&01001ENQENQ
00111U701010R4
01100N,01011JBEL
01101F!01110C:
01111K)10001Z+
10000T510010L)
10011W210011W2
Controls include (CR, 01000), line feed (LF, 00010), (SP, 00100), null (NUL, 00000), enquiry (ENQ, 01001), and bell (BEL, 01011 in FIGS). ITA2 incorporated enhancements like the bell for alerts and basic error detection via repeated garbled characters, with ENQ for retransmission requests, improving reliability without altering the 5-bit structure. The United States Teletype (TTY) variant of ITA2 included minor remappings for American English, such as swapping and # (e.g., 10100 as in ITA2 becomes # in TTY FIGS), and reassigning apostrophe and bell for local needs like the dollar sign.

Specialized Variants

In the 1920s and 1930s, meteorologists adapted International Telegraph Alphabet No. 2 (ITA2) into a specialized variant for weather reporting, to transmit synoptic observations over telegraph and teletype networks. This weather code remapped figures-mode symbols—retaining digits and controls—to represent meteorological data like wind directions and cloud cover. For instance, letters in figures shift were reassigned to arrows (e.g., A for north/upward; F for east/rightward) and cloud symbols (e.g., C for clear, circle; B for overcast, crossed circle), replacing punctuation to prioritize shorthand. These maintained the 5-bit structure but altered assignments for international weather services, with hourly transmissions at 60 words per minute on modified teletype models like the Model 15. During World War II, Baudot-based variants integrated into cipher machines for encrypted teletype. The German Lorenz SZ40/42 processed Baudot , XORing with a keystream from 12 wheels for transmission via radio or wire. The U.S. (ECM Mark II) used Baudot input/output, permuting 32 combinations with rotor banks for a keyspace over 10^26, resisting . Adaptations used custom shifts to align with machine alphabets but retained core patterns for compatibility. Other extensions included ARQ protocols for radio teletypes (RTTY) in the late 1940s, appending parity/ to 5-bit characters for error detection and retransmission requests over noisy HF channels, improving reliability in /military use without base encoding changes. Early ARQ used selective repeat for corrupted blocks. These variants had limited adoption, mainly in weather and military until the , when 7-bit ASCII supplanted them for greater flexibility.

Applications

Telegraphy Systems

Baudot code was integral to early mechanical telegraph networks, enabling the transmission of multiple messages over a single wire through synchronous time-division multiplexing. In 1874, Émile Baudot developed a printing telegraph system that utilized a five-unit code to distribute signals across 2 to 6 channels simultaneously, with standard configurations supporting four operators on one line. This multiplex setup, incorporating a rotating distributor for synchronization, marked a significant advancement in telegraph hardware by allowing efficient sharing of bandwidth without interference. Operational workflows in Baudot-based telegraphy involved operators using a specialized keyboard to generate five-bit pulses representing characters, transmitted at a steady rhythm of approximately 180 characters per minute. The sender's distributor connected the keyboard to the line, while the receiver employed electromagnets to decode the pulses and print text directly on paper tape or rolls, eliminating manual transcription. Synchronization was maintained via correcting currents and cadence signals, ensuring reliable decoding across the network. Key devices like the Creed teleprinter, introduced in the 1920s, exemplified Baudot's application in advanced hardware, using the code for page printing at speeds of 60 words per minute. Compared to , Baudot's fixed-length five-bit structure facilitated automation by standardizing transmission durations, reducing operator fatigue and enabling mechanized handling that achieved up to 100 words per minute in later systems. Baudot code powered global telegraph networks from the to the , with widespread adoption in French lines starting in 1875, British Post Office circuits by 1897, and U.S. systems through variants like the Murray code for services such as . These networks handled high-volume international traffic, supporting economic and diplomatic communications until phased out by more advanced encodings.

Teleprinters and Early Data Transmission

The Teletype Model 15, introduced in the 1930s, and its successor the Model 19 in the 1940s, were pivotal asynchronous teleprinters that employed the International Telegraph Alphabet No. 2 (ITA2), a variant of the Baudot code, for text transmission and reception. These machines operated at a standard speed of 45.5 baud, utilizing five data bits per character along with a start bit and 1.5 stop bits to synchronize the serial asynchronous communication, enabling reliable standalone and over leased lines or direct connections. The models featured mechanical keyboards and typing units that punched and read 5/8-inch paper tape, facilitating offline message preparation and automated transmission, which became essential for business and news services through the 1950s. Starting in the , with significant military adoption in the , Baudot code was adapted for radio teletype (RTTY) systems, marking a significant extension to high-frequency (HF) radio communications, particularly in military and applications. RTTY employed (FSK) to modulate the Baudot ITA2 signals onto radio carriers, allowing text transmission over despite and ; military forces, including the U.S. Army, used diversity reception setups post-World War II to enhance reliability in challenging conditions. operators began experimenting with surplus teleprinters in 1946, achieving the first RTTY contacts that year, which popularized the mode for long-distance messaging among hobbyists into the following decades. Baudot code interfaced with early computers in the and , serving as the encoding for paper tape peripherals on some systems. For data transmission in environments, Baudot code integrated seamlessly with readers and punches, enabling automated loading of programs and datasets into computers or networks at speeds of 45 to 75 . This setup allowed for efficient handling of sequential jobs in scientific and administrative workflows, where tape reels stored thousands of characters for unattended operation, though limited by the code's 32-character sets requiring shift mechanisms. By the 1960s, Baudot code in teleprinters began to decline as 7-bit ASCII emerged as the new standard, offering expanded character support and compatibility with computing advancements; the Teletype Model 33, released in 1963, fully transitioned to ASCII at 110 baud, rendering Baudot-based models obsolete for most commercial and data applications.

Legacy

Influence on Later Encodings

The Baudot code served as a foundational precursor to the American Standard Code for Information Interchange (ASCII), adopted in 1963, by establishing a 5-bit fixed-length binary structure for encoding text and control characters, which ASCII expanded to 7 bits to support 128 characters while retaining concepts like shift mechanisms for mode switching between letter and figure sets. This evolution addressed the limitations of Baudot's 32-symbol capacity, enabling broader international compatibility in data processing and telecommunications. Baudot's framework directly influenced subsequent CCITT (now ) standards, forming the basis for International Telegraph Alphabet No. 3 (ITA3) and No. 5 (ITA5) in the , which transitioned from 5-bit to 7-bit encodings to accommodate more symbols and controls while preserving the synchronous transmission principles. These developments culminated in ISO/IEC 646, the international standard for 7-bit character codes, which adopted Baudot-derived variants for national adaptations, ensuring in global and early computing networks. As the first widely adopted fixed-length binary encoding for textual data, Baudot pioneered uniform symbol representation that inspired later systems like Hollerith punched-card codes for tabulating machines and IBM's Extended Binary Coded Decimal Interchange Code (EBCDIC), which built on binary principles for mainframe data interchange despite diverging into 8-bit formats. This shift marked a progression from telegraphy-specific codes to versatile computing standards, emphasizing error-resistant binary transmission. The shift mechanism in Baudot, using dedicated characters to toggle between alphabetic and numeric modes, left a lasting legacy in design, influencing mode-switching concepts explored in early drafts and modern escape sequences for multilingual support in standards like ISO 2022. Such techniques allowed efficient use of limited bit widths, a principle echoed in protocols requiring dynamic character set changes without fixed-state assumptions. By the 1950s, Baudot began yielding to 6- and 7-bit codes in teleprinter systems, as evidenced by the adoption of ASCII precursors that offered expanded repertoires and parity bits for reliability, phasing out 5-bit limitations in favor of scalable international standards. This transition facilitated the integration of teleprinters into computing environments, bridging telegraphy to digital data transmission.

Modern and Historical Relevance

Despite the dominance of modern character encodings like ASCII, the Baudot code maintains niche relevance in operations, particularly through (RTTY) modes used in contests and long-distance communications (). As of 2025, RTTY transmissions on shortwave frequencies continue to employ the 5-bit Baudot (also known as ITA2) with (FSK) at speeds such as 45.45 baud, enabling keyboard-to-keyboard text exchange among radio enthusiasts. Software emulators like facilitate this by decoding and encoding Baudot signals in real-time, supporting its use in events like the ARRL RTTY Roundup without requiring vintage hardware. This persistence stems from RTTY's simplicity and low bandwidth requirements, though it is gradually supplemented by more efficient digital modes. In historical , Baudot code underpins emulations of II-era machines such as the German Lorenz SZ42 and British Typex, preserved and demonstrated in museums for educational purposes. The , for instance, encrypted Baudot-Murray code streams from teleprinters, and virtual emulators now allow interactive simulations of its operation, replicating wartime encryption processes. Institutions like The National Museum of Computing in host these digital recreations, including online tools for enciphering messages, which extend to WWII reenactments and workshops. Such efforts highlight Baudot's role in high-level , with physical artifacts and software models ensuring its operational legacy. Preservation initiatives sustain Baudot-encoded artifacts in digital archives, where punched paper tapes and telegrams from the telegraph era are scanned and cataloged for scholarly access. Libraries and national repositories use specialized tape readers to convert Baudot perforations into readable text, preventing data loss from deteriorating media. These efforts include projects digitizing historical teleprinter outputs, making them searchable in formats compatible with contemporary systems, such as the Centrum Wiskunde & Informatica's (CWI) preservation of its punched tape collection as of 2025. In rare contemporary niches, Baudot appears in legacy interfaces for vintage computing restoration and occasional IoT adaptations tied to old industrial equipment. Modern restorations employ emulators to interface these with current hardware, as seen in hobbyist projects linking vintage teletypes to microcontrollers like the . Such uses are limited to preservation and experimental contexts, with no widespread adoption in IoT due to inefficiency compared to 7- or 8-bit standards. Culturally, Baudot code features in media exploring and early , underscoring its foundational influence without prospects for mainstream revival amid ASCII's prevalence. Documentaries and books on communication evolution, such as those detailing the shift from Morse to binary codes, portray Baudot as a pivotal step in data transmission, often in narratives of . Its legacy endures in these depictions as a precursor to modern encodings, briefly shaping ASCII's shift mechanisms for case and symbols.

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