doubleletter

Double letter, also called a doubled letter or geminate, refers to two identical letters appearing consecutively within a word. Examples include letter (tt), committee (mm and tt), book (oo), and cool (oo). The phenomenon occurs in many languages that use the Latin script and in other scripts with similar orthographic conventions.

In linguistics, doubled letters frequently signal gemination—phonetic lengthening or stronger articulation of a consonant. In languages such as Italian, a doubled consonant is pronounced with a longer or more forceful articulation (as in gatto). In Finnish and Swedish, doubled vowels or consonants can indicate length. In English, doubled letters often reflect spelling and morphology rather than phonemic length; for example, committee or happiness.

Orthography and morphology: Doubling can arise from word formation, such as affixation or compounding; it can also reflect historical pronunciation that has since changed. Some languages treat certain doubled sequences as digraphs or as a single letter, which affects sorting, indexing, and other lexical rules.

Computational considerations: Text-processing tasks such as search, sorting, hyphenation, and spell-check require handling of doubled letters; normalizing or preserving doubles can affect results. Word games and typography rules also consider doubled letters in scoring and validation.

See also: gemination, double consonant, digraph, vowel length, orthography.