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Ŭ or ŭ is a letter in the Esperanto alphabet, and is also used in the Belarusian language, when written in the 20th-century form of the Belarusian Latin alphabet. The accent mark (diacritic) is known as a breve.
The letter ŭ is called non-syllabic u (romanised: u nieskładovaje) in Belarusian because it resembles the vowel u but forms no syllables. It is an allophone of /v/ that forms the diphthongs aŭ, eŭ, oŭ and is equivalent to [u̯]. Its Cyrillic counterpart is ў. Sometimes (as in National Geographic atlases), the Cyrillic letter ў is Romanized as w.
Ŭ represents a semivowel in the orthography of Esperanto, which is an international auxiliary language publicly presented in 1887. As in Belarusian, Esperanto Ŭ is pronounced as a non-syllabic [u̯], primarily in the diphthongs aŭ and eŭ.
Ŭ may also be used for [w] in foreign names, such as Ŭaŝingtono for "Washington", although it usually is written with v (Vaŝingtono). It is also used for [w] in onomatopoeias, as in ŭa! "waa!", and uniquely in one native lexical word, ŭo, which is the Esperanto name of the letter ŭ itself.
Other uses 
Several schemes for pronunciation of English words have used "ŭ". For example, The American Heritage Dictionary of the English Language has used "ŭ" for /ʌ/, the vowel in the English word "cut".
See also 
- S. Young (2006) "Belorussian". In the Encyclopedia of language and linguistics, 2nd ed.
- Булыка (Bulyka). У нескладовае // Энцыклапедыя літаратуры і мастацтва Беларусі. Т.4. p.377.
Letter U with diacritics
Letters using breve sign ( ◌̆ )