Two dots (diacritic)

Diacritical marks of two dots ¨, placed side-by-side over or under a letter, are used in several languages for several different purposes. The most familiar to English-language speakers are the diaeresis and the umlaut, though there are numerous others. For example, in Albanian, ë represents a schwa. Such diacritics are also sometimes used for stylistic reasons (as in the family name Brontë or the band name Mötley Crüe).

In modern computer systems using Unicode, the two-dot diacritics are almost always encoded identically, having the same code point. For example, U+00F6 ö LATIN SMALL LETTER O WITH DIAERESIS represents both o-umlaut and o-diaeresis. Their appearance in print or on screen may vary between typefaces but rarely within the same typeface.

The word trema (French: tréma), used in linguistics and also classical scholarship, describes the form of both the umlaut diacritic and the diaeresis rather than their function and is used in those contexts to refer to either.

As the "diaeresis" diacritic, it is used to mark the separation of two distinct vowels in adjacent syllables when an instance of diaeresis (or hiatus) occurs, so as to distinguish from a digraph or diphthong. For example, in the obsolete spelling coöperate, the diaeresis reminded the reader that the word has four syllables co-op-er-ate, not three. It is used in several languages of western and southern Europe, though rarely now in English. One well-known usage is in French - the diaeresis is used in naïve, which is commonly spelled in English without the diaeresis. It is, however, obligatory in French, to show that it is pronounced [na.iv] rather than [nɛv].

As the "umlaut" diacritic, it indicates a sound shift  – also known as umlaut – in which a back vowel becomes a front vowel. It is a specific feature of German and other Germanic languages, affecting the graphemes ⟨a⟩, ⟨o⟩, ⟨u⟩ and ⟨au⟩, which are modified to ⟨ä⟩, ⟨ö⟩, ⟨ü⟩ and ⟨äu⟩.

It can be seen in the Sütterlin script, formerly used widely in German handwriting, in which the letter e is formed as two short parallel vertical lines very close together (see under Sütterlin#Characteristics).

The two dot diacritic is also sometimes used for purely stylistic reasons. For example, the Brontë family's surname was derived from Gaelic and had been anglicised as "Prunty", or "Brunty", but at some point, the father of the sisters, Patrick Brontë (born Brunty), decided on the alternative spelling with a diaeresis diacritic over the terminal ⟨e⟩ to indicate that the name had two syllables.

Similarly, the "metal umlaut" is a diacritic that is sometimes used gratuitously or decoratively over letters in the names of hard rock or heavy metal bands – for example, those of Motörhead and Mötley Crüe, and of parody bands, such as Spın̈al Tap.