It’s strange: you can learn something, use it daily for years, and never notice the underlying patterns. Until I read a post on Paleoglot this morning, I hadn’t realised how regular six of the Japanese numerals are.

The rule to double is: iu, oa. (Keep in mind that h and f represent the same morpheme in Japanese.)

1 ひとつ hitotsu 2 ふたつ futatsu
3 みっつ mittsu 6 むっつ muttsu
4 よっつ yottsu 8 やっつ yattsu

The pattern has disappeared from the basic forms of the numbers between Old and Modern Japanese, but it’s still represented perfectly in the counter forms listed above.