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U (named u /ˈjuː/, plural ues) is the 21st letter and the fifth vowel in the ISO basic Latin alphabet.

The letter u ultimately comes from the Phoenician letter Waw by way of the letter y. See the letter y for details.

During the late Middle Ages, two forms of 'v' developed, which were both used for its ancestor 'u' and modern 'v'. The pointed form 'v' was written at the beginning of a word, while a rounded form 'u' was used in the middle or end, regardless of sound. So whereas 'valour' and 'excuse' appeared as in modern printing, 'have' and 'upon' were printed 'haue' and 'vpon'. The first recorded use of 'u' and 'v' as distinct letters is in a Gothic alphabet from 1386, where 'v' preceded 'u'. Printers eschewed capital 'U' into the 17th century and the distinction between the two letters was not fully accepted by the French Academy until 1762.

In English, the letter ⟨u⟩ has four main pronunciations. There are "long" and "short" pronunciations. Short ⟨u⟩, found originally in closed syllables, most commonly represents /ʌ/ (as in 'duck'), though it retains its old pronunciation /ʊ/ after labial consonants in some words (as in 'put') and occasionally elsewhere (as in 'sugar'). Long ⟨u⟩, found originally in words of French origin (the descendent of Old English long u was respelled as ⟨ou⟩), most commonly represents /juː/ (as in 'mule'), reducing to /uː/ after ⟨r⟩ (as in 'rule') and sometimes (or optionally) after ⟨l⟩ (as in 'lute'), and after additional consonants in American English (see do–dew merger). (After ⟨s⟩, /sjuː, zjuː/ have assimilated to /ʃuː, ʒuː/.) In a few words, short ⟨u⟩ represents other sounds, such as /ɪ/ in 'business' and /ɛ/ in 'bury'.

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