The colon alphabetic letter ꞉ is used in a number of languages and phonetic transcription systems, for vowel length in Americanist Phonetic Notation, for the vowels ⟨a꞉⟩ [ɛ] and ⟨o꞉⟩ [ɔ] in a number of languages of Delhi, India, and for grammatical tone in several languages of Africa. It resembles but differs from the colon punctuation mark, :. In some fonts, the two dots are placed a bit closer together than those of the punctuation colon so that the two characters are visually distinct. In Unicode it has been assigned the code U+A789 ꞉ MODIFIER LETTER COLON, which behaves like a letter rather than a punctuation mark in electronic texts. In practice, however, an ASCII colon is frequently used for the letter.

In Windows and macOS, the letter colon can be used to emulate the punctuation colon in file names, where the punctuation colon is a reserved character that cannot be used.

Several of the Native American languages of North America use the colon to indicate vowel length. Zuni is one. Other languages include Hupa of California, Oʼodham of Arizona, Sayula Popoluca of Mexico and Mohawk of Ontario. Still others use a half colon (just the top dot of the colon, or a middot, U+A78F ꞏ LATIN LETTER SINOLOGICAL DOT). Both conventions derive from Americanist phonetic notation (below).

The colon is used as a grammatical tone letter in Budu in the Democratic Republic of the Congo, in Sabaot in Kenya, and in some Grebo in Liberia. It is used for the vowels /ɛ/ and /ɔ/ in several languages of Papua New Guinea: Erima, Gizra, Go꞉bosi, Gwahatike, Kaluli, Kamula, Kasua, Kuni-Boazi and Zimakani.^{[1][Not clear that it does the same thing in all those languages. E.g. what is the <e꞉> of Boazi?]}

In Americanist phonetic notation, a colon may be used to indicate vowel length. This convention is somewhat less common than the half-colon.

In the International Phonetic Alphabet, a special triangular colon-like letter is used to indicate that the preceding consonant or vowel is long. Its form is that of two triangles pointing toward each other rather than the two dots of Americanist notation. It is available in Unicode as U+02D0 ː MODIFIER LETTER TRIANGULAR COLON. If the upper triangle is used without the lower one (U+02D1 ˑ MODIFIER LETTER HALF TRIANGULAR COLON), it designates a half-long vowel or consonant.^[2]

The Uralic Phonetic Alphabet uses U+02F8 ˸ MODIFIER LETTER RAISED COLON.^[3]