U (Cyrillic)

It commonly represents the close back rounded vowel /u/, somewhat like the pronunciation of ⟨oo⟩ in "boot" or "rule".

Historically, Cyrillic U evolved as a specifically East Slavic short form of the digraph ⟨оу⟩ used in ancient Slavic texts to represent /u/.

The digraph was itself a direct loan from the Greek alphabet, where the combination ⟨ου⟩ (omicron-upsilon) was also used to represent /u/.

(The letter Izhitsa was removed from the Russian alphabet in the orthography reform of 1917/19.)

In Tuvan the Cyrillic letter can be written as a double vowel.

U , from Alexandre Benois ' 1904 alphabet book . It shows Ulitsa (street) and uraganʺ ( hurricane ).
A PFM-1 training mine, distinguishable from the live version by the presence of the letter У (short for учебный, uchebnyy , "for training").
Similarity with Y (uppercase): The grapheme on the left is clearly a Cyrillic U, the one in the middle may represent both letters, the one on the right is clearly a Greek or Latin Y.