|Cyrillic letter Uk|
|Cyrillic numerals: 400|
|List of Cyrillic letters|
Uk (Ѹ ѹ; italics: Ѹ ѹ) is a letter of the early Cyrillic alphabet. It was originally a digraph of the Cyrillic letters O (О о) and U (У у)[dubious ] or less frequently O and Izhitsa (Ѵ ѵ). To save space, it was often written as a vertical ligature (Ꙋ ꙋ), called "monograph Uk". In modern times, ⟨оу⟩ has been replaced by the simple ⟨у⟩.
In later manuscripts, the digraph Uk has three cases: lowercase ⟨ѹ⟩, uppercase ⟨Ѹ⟩, and an all-caps form which is used in all-capitals titles ⟨ОУ⟩.
Borrowing from Greek 
Both the horizontal and the vertical digraph were borrowed from the Greek alphabet. The Greek ligature Ou (Ȣ ȣ) is frequently found in Greek medieval manuscripts and in some modern editions of classical texts. Modern Greek still uses ⟨ου⟩ (omicron-upsilon) for /u/ but rarely uses the vertical ligature.
Development of the use of Uk in Old East Slavic 
The simplification of the ligature ⟨ѹ⟩ to ⟨у⟩ was first brought about in Old East Slavic texts and only later taken over into South Slavic languages.
One can see this development in the Novgorod birch-bark letters: The degree to which this letter was used here differed in two positions: in word-initial position or before a vowel (except for the jers), and after a consonant.
Before a consonant, ⟨ѹ⟩ was used 89% of the time in the writings before 1100. By 1200, it was used 61% of the time, with the letter ⟨у⟩ used 14% of the time; by 1300, ѹ had reached 28%, surpassed by ⟨у⟩ at 45%. From the late 14th century on, there are no more instances of ⟨ѹ⟩ being used in this position, with ⟨у⟩ appearing 95% of the time.
The decrease in usage was more gradual after a consonant. Although there are no instances of the use of ⟨у⟩ in this position before c. 1200, ⟨ѹ⟩ gradually decreased from 88% before 1100 to 57% by 1200. The frequency of ⟨ѹ⟩ remained steady between 47% and 44% until 1400, when it experienced another decrease to 32%. Meanwhile, the use of ⟨у⟩ increased from 4% in the early 13th century, to 20% by the mid-13th century, 38% by the mid 14th century, and 58% by the early 15th century.
Representation on computers 
The letter Uk was first represented in Unicode 1.1.0 as U+0478 and 0479, CYRILLIC CAPITAL/SMALL LETTER UK (Ѹ ѹ). It was later recognized that the glyph to be used for the letter had not been adequately specified, and it had been represented as either a digraph or monograph letter in different released fonts. There was also the difficulty that in written texts the letter may appear in lowercase (ѹ), uppercase (Ѹ), or in all caps (ОУ), which was not allowed for.
To resolve this ambiguity, Unicode 5.1 has deprecated the use of the original code points, introduced the new U+A64A and A64B, CYRILLIC CAPITAL/SMALL LETTER MONOGRAPH UK (Ꙋ ꙋ), and recommends composing the digraph with two individual characters ⟨о⟩+⟨у⟩ .
However, the recommended method may cause some text representation problems. Actually there was not modern letter У in the Old Church Slavonic orthography, and its code point was replaced in different Old Slavonic computer fonts with digraph or monograph forms of the Uk or with the tailed form of Izhitsa. Tailed Izhitsa may be used as a part of the digraph, but using the shape of the monograph Uk as a part of the digraph Uk is incorrect.
Computing codes 
|Unicode name||CYRILLIC CAPITAL LETTER UK||CYRILLIC SMALL LETTER UK||CYRILLIC CAPITAL LETTER
|CYRILLIC SMALL LETTER
|UTF-8||209 184||D1 B8||209 185||D1 B9||234 153 138||EA 99 8A||234 153 139||EA 99 8B|
|Numeric character reference||Ѹ||Ѹ||ѹ||ѹ||Ꙋ||Ꙋ||ꙋ||ꙋ|
- Everson, Michael; et al. (2007). "Proposal to encode additional Cyrillic characters in the BMP of the UCS" (application/pdf).
- Kaplan, Michael S. “Every character has a story #10: U+0478/U+0479 (CYRILLIC LETTER UK)”, May 21, 2005.
- Zaliznyak, Andrey (2004). Drevnenovgorodskij dialekt. Moscow: Jazyki slavjanskoj kul'tury.