Such vowels are signified in phonetic transcription by a colon. In many languages a long vowel may contrast with a short vowel; for example, in Finnish, [tuli] (spelled "tuli") means "fire" while [tu:li] (spelled "tuuli") means "wind."
English has long and short vowels, but they do not contrast in this way. English vowel length is fairly regular, however: vowels are long before voiced consonants in the coda of a syllable. For example, the vowel ([æ]) in [bæt] ("bat") is long, because /t/ is unvoiced, while the same vowel in [bæ:d] ("bad") is long, because /d/ is voiced.
English vowels can also be said to have natural length. [æ] is naturally long, before it is lengthened further by voiced consonants, while a vowel such as [I] is short. Vowel stress in English is also related to length, as [e], [i], [u], and other stressed vowels are also naturally long.
Vowels length can also be signified by a half-colon (a colon with only the top dot), meaning half-long, and a double colon, meaning twice as long as a regular vowel.