Alphabetical order is a system whereby character strings are placed in order based on the position of the characters in the conventional ordering of an alphabet. It is one of the methods of collation. In mathematics, a lexicographical order is the generalization of the alphabetical order to other data types, such as sequences of numbers or other ordered mathematical objects.

When applied to strings or sequences that may contain digits, numbers or more elaborate types of elements, in addition to alphabetical characters, the alphabetical order is generally called a lexicographical order.

To determine which of two strings of characters comes first when arranging in alphabetical order, their first letters are compared. If they differ, then the string whose first letter comes earlier in the alphabet comes before the other string. If the first letters are the same, then the second letters are compared, and so on. If a position is reached where one string has no more letters to compare while the other does, then the shorter string is deemed to come first in alphabetical order.

Capital or upper case letters are generally considered to be identical to their corresponding lower case letters for the purposes of alphabetical ordering, although conventions may be adopted to handle situations where two strings differ only in capitalization. Various conventions also exist for the handling of strings containing spaces, modified letters, such as those with diacritics, and non-letter characters such as marks of punctuation.

The result of placing a set of words or strings in alphabetical order is that all of the strings beginning with the same letter are grouped together; within that grouping all words beginning with the same two-letter sequence are grouped together; and so on. The system thus tends to maximize the number of common initial letters between adjacent words.

The order of the letters of the alphabet is attested from the 14th century BC in the town of Ugarit on Syria's northern coast. Tablets found there bear over one thousand cuneiform signs, but these signs are not Babylonian and there are only thirty distinct characters. About twelve of the tablets have the signs set out in alphabetic order. There are two orders found, one of which is nearly identical to the order used for Hebrew, Greek and Latin, and a second order very similar to that used for Geʽez.

It is not known how many letters the Proto-Sinaitic alphabet had nor what their alphabetic order was. Among its descendants, the Ugaritic alphabet had 27 consonants, the South Arabian alphabets had 29, and the Phoenician alphabet 22. These scripts were arranged in two orders, an ABGDE order in Phoenician and an HLĦMQ order in the south; Ugaritic preserved both orders. Both sequences proved remarkably stable among the descendants of these scripts.

As applied to words, alphabetical order was first used in the 1st millennium BCE by Northwest Semitic scribes using the abjad system. However, a range of other methods of classifying and ordering material, including geographical, chronological, hierarchical and by category, were preferred over alphabetical order for centuries.