Hidden patterns in Japanese counting

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: i → u, o → a. (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.