In English, Yu is commonly romanized as ⟨yu⟩. In turn, ⟨ю⟩ is used, where is available, in transcriptions of English letter ⟨u⟩ (in open syllables), and also of the ⟨ew⟩ digraph. The sound [y], like ⟨u⟩ in French and ⟨ü⟩ in German, may also be approximated by the letter ⟨ю⟩.
It is a so-called iotated vowel, pronounced in isolation as /ju/, like the pronunciation of ⟨you⟩ in "youth". After a consonant, no distinct [j] sound is pronounced, but the consonant is softened. The exact pronunciation of the vowel sound of ⟨ю⟩, in Russian depends also on the succeeding sound because of allophony. Before a soft consonant, it is [ʉ], the close central rounded vowel, as in 'rude'. If a hard consonant ⟨ю⟩ or nothing or at the end of a word, the result is a back vowel [u], as in 'Lewis'.
Apart from the form I-O, in early Slavonic manuscripts the letter appears also in a mirrored form O-I (Ꙕ, ꙕ). It is the latter form that is probably the original, precisely displaying the Greek combination omicron-iota (οι). At the time that the Greek alphabet was adapted to the Slavonic language giving rise to the Cyrillic alphabet, it denoted the close front rounded vowel/y/ in educated Greek speech. This digraphic representation of /y/ was so basic for speakers of Greek that the simple letter upsilon (υ) representing the same sound came to be called υ ψιλόν (y psilon) "simple" υ in contrast to "complex" οι. The close front rounded vowel does not appear in East Slavic. See above.
There was another way for it to lead to the modern form. By the analogy to several 'iotated' letters Ѥ, ІА, Ѩ and Ѭ, the ancient ligature (or letter) Uk ⟨оѵ⟩/⟨оу⟩ possibly had its iotated form ⟨іоѵ⟩/⟨іоу⟩.
Also, the iotified big Yus ⟨Ѭ⟩ merged itself to ⟨ю⟩ in East Slavic languages.
Related letters and other similar characters