Why is 10日 reading とおか and not とうか?

Howdy howdy. The 10日/十日 vocabulary term is kicking my butt. I know it has a long vowel — and, every time, I’m tempted to spell it とうか. But WaniKani (and other sources is ) have the correct reading/spelling as とおか.

I find this additionally strange because 8日 = ようか with 「おう」. Maybe there’s an etymology difference behind 八 and 十?

Of course we have exceptions, like おおき/大, to the long-お-vowel-takes-う “rule” (東京/とうきょう, 王/おう, 学校/がっこう). But is that right? Is that even a rule? Is 大き the exception or is there another dependency? And can I get a durtle badge for this?

ようか actually used to be やうか long ago, but it shifted, so they updated the spelling, but there was no reason to also change the う.

Generally speaking any time an お extends an お sound, it’s a kunyomi. う extends an お sound in onyomi. It’s just a historical spelling thing. Of course, ようか ends up being an exception to that since it took a different path.

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Thanks much. やうか makes sense given やっつ as well. Did not know this about kunyomi long vowels — onward!

image

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kunyomi long-おs generally come from historical spelling shifts, AIUI most commonly from ほ. in 十日’s case it seems to have originally been とをか.

sources:
https://furigana.info/w/十日

the second link points out that 七日 also used to be なぬか

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Twin Peaks

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One day I will be able to read this. Cheers!

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