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VOWEL QUANTITY. A term in phonetics and poetics for the length of a VOWEL, usually indicated in phonetic transcription by a LENGTH MARK [ː] or a colon [:] after a vowel, as in /aː/. Vowels so marked have in general greater duration than the same vowels with no such mark. Vowels so marked are described as long, and unmarked vowels are short, a distinction known as vowel length. However, the measurable duration of vowels depends also on at least two other factors: (1) Vowel height, in terms of the position of the tongue. Open vowels as in ban/ban/ or balm /bɑːm/ are longer than close vowels as in bin/bɪn/ or beam /biːm/. (2) Environment, in terms of preceding and following sounds. Vowels are shortened before some consonants and lengthened before others, for example /uː/ is longer in move /muːv/ than in boot /buːt/. These factors have different weightings in different varieties of English, and contribute to the variety of rhythms in English. In many varieties, short vowels may have greater duration than long vowels: for example, /a/ in jazz may be longer than /iː/ in sleep. To avoid the confusion, some phoneticians consider it preferable to treat the length mark as a mark of quantity rather than duration, and refer to ‘heavy’ and ‘light’ vowels. If a vowel has sufficient duration, as in halve /haːv/, there is time for the organs that form it to move into their target positions and remain there briefly before moving to the next target. Such a vowel is described as tense. If the vowel is too short, the organs have to leave the target as soon as they reach it, and in extreme cases (for example, in six /sɪks/) may not reach the target at all. Such a vowel is described as lax. In view of the time required to move to more peripheral vowel positions, tense vowels tend to be peripheral and lax vowels closer to schwa, the neutral or central vowel. If an English word ends in a vowel, then the vowel must be either heavy (that is, it is either marked with a length mark or is a diphthong) or reduced: for example, me /miː/, day /deɪ/, banana /-nə/. See VOWEL QUALITY.