It's not just historical. "Ů" is phonologically a longer "u", but it is actually an alternation [1] of "o": for example see how nominative "dům" becomes "domu"

It's not just historical. "Ů" is phonologically a longer "u", but it is actually an alternation [1] of "o": for example see how nominative "dům" becomes "domu" in the genitive (likewise "stůl", "bůh", etc.). When a "u" becomes longer, instead, it becomes a "ú" as the first letter of the word, but otherwise it becomes "ou"; for example see how the feminine nominative of (some) nouns and adjectives is "a" and "á" respectively, while the accusative is "u" and "ou", or how the perfective companion of "kupovat" is "koupit". (Also, see how I sneaked in a "Ů" in the second sentence :)). [1] https://en.wikipedia.org/wiki/Alternation_%28linguistics%29 https://en.wikipedia.org/wiki/Alternation_%28linguistics%29