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〉.
Apart from the form I-O, in early Slavonic manuscripts the letter appears also in a mirrored form O-I (Ꙕ, ꙕ). It is this form that is probably the original,[vague] 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), this 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. Note that the close front rounded vowel unlikely appears in East Slavic, see above.
There was another way, which may 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