Apart from the form "I-O", in early Slavonic manuscripts the letter appears also in a mirrored form "O-I" (). 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 in educated Greek speech. This digraphic representation of 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.