Uk (Cyrillic)

Numeric value:	400
Slavic letters
А Б В Г Ґ Д Ђ Ѓ Е Ѐ Ё Є Ж З Ѕ И Ѝ І Ї Й Ј К Л Љ М Н Њ О П Р С Т Ћ Ќ У Ў Ф Х Ц Ч Џ Ш Щ Ъ Ы Ь Э Ю Я
Non-Slavic letters
Ӑ А̄ А̊ А̃ Ӓ Ӓ̄ Ә Ә́ Ә̃ Ӛ Ӕ Ғ Г̧ Г̑ Г̄ Ҕ Ӻ Ӷ Ԁ Ԃ Ꚃ Ꚁ Ꚉ Ԫ Ԭ С́ Ӗ Е̄ Е̃ Ё̄ Є̈ Ӂ Җ Ꚅ Ӝ Ԅ Ҙ Ӟ З́ Ԑ Ԑ̈ Ӡ Ԇ Ӣ И̃ Ҋ Ӥ Қ Ӄ Ҡ Ҟ Ҝ Ԟ Ԛ Ӆ Ԯ Ԓ Ԡ Ԉ Ԕ Ӎ Ӊ Ң Ԩ Ӈ Ҥ Ԣ Ԋ О̆ О̃ О̄ Ӧ Ө Ө̄ Ӫ Ҩ Ԥ Ҧ Р̌ Ҏ Ԗ Ҫ Ԍ Ꚑ Ҭ Ꚋ Ꚍ Ԏ У̃ Ӯ Ӱ Ӱ́ Ӳ Ү Ү́ Ұ Х̑ Ҳ Ӽ Ӿ Һ Һ̈ Ԧ Ꚕ Ҵ Ꚏ Ҷ Ӵ Ӌ Ҹ Ꚓ Ꚗ Ꚇ Ҽ Ҿ Ы̆ Ы̄ Ӹ Ҍ Э̆ Э̄ Э̇ Ӭ Ӭ́ Ӭ̄ Ю̆ Ю̈ Ю̈́ Ю̄ Я̆ Я̄ Я̈ Ԙ Ԝ Ӏ
Archaic letters
Ꙁ Ꙃ Ꙅ Ꙇ Ꙉ Ҁ Ѻ ОУ Ꙋ Ѡ Ѽ Ꙍ Ѿ Ꙏ Ѣ Ꙑ Ꙓ Ꙕ Ꙗ Ѥ Ѧ Ꙙ Ѫ Ꙛ Ѩ Ꙝ Ѭ Ѯ Ѱ Ѳ Ѵ Ѷ Ꙟ Ꙡ Ꙣ Ꙥ Ꙧ Ꙩ Ꙫ Ꙭ ꙮ Ꚙ Ꚛ
List of Cyrillic letters Cyrillic digraphs

Uk (Ѹ ѹ; italics: Ѹ ѹ) is a digraph of the early Cyrillic alphabet, although commonly considered and used as a single letter. It is a combination of the Cyrillic letters О and У or less frequently O and 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 In modern times, ⟨оу⟩ has been replaced by the simple ⟨у⟩.

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.

The simplification of the digraph ⟨оу⟩ 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, ⟨оу⟩ 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, оу had reached 28%, surpassed by ⟨у⟩ at 45%. From the late 14th century on, there are no more instances of ⟨оу⟩ 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, ⟨оу⟩ gradually decreased from 88% before 1100 to 57% by 1200. The frequency of ⟨оу⟩ 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.

...
Wikipedia