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. Wikipedia