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In linguistics, vowel length is the perceived length of a vowel sound: the corresponding physical measurement is duration. In some languages vowel length is an important phonemic factor, meaning vowel length can change the meaning of the word, for example in Arabic, Estonian, Finnish, Fijian, Japanese, Kannada, Kyrgyz, Latin, Malayalam, Old English, Scottish Gaelic, Tamil and Vietnamese.

While vowel length alone does not change word meaning in most dialects of modern English, it is said to do so in a few dialects, such as Australian English, Lunenburg English, New Zealand English, and South African English. It also plays a lesser phonetic role in Cantonese, unlike in other varieties of Chinese, which do not have phonemic vowel length distinctions.

Many languages do not distinguish vowel length phonemically, meaning that vowel length does not change meaning. However, the amount of time a vowel is uttered can change based on factors such as the phonetic characteristics of the sounds around it, for instance whether the vowel is followed by a voiced or a voiceless consonant.

Languages that do distinguish vowel length phonemically usually only distinguish between short vowels and long vowels. Very few languages distinguish three phonemic vowel lengths; some that do so are Estonian, Luiseño, and Mixe. However, languages with two vowel lengths may permit words in which two adjacent vowels are of the same quality: Japanese hōō, "phoenix", or Ancient Greek ἀάατος [a.áː.a.tos], "inviolable". Some languages that do not ordinarily have phonemic vowel length but permit vowel hiatus may similarly exhibit sequences of identical vowel phonemes that yield phonetically long vowels, such as Georgian გააადვილებ [ɡa.a.ad.vil.eb], "you will facilitate it".