noun
long 'o' vowel from 'a'+'u' combination
Linguistic term referring to the historical sound change where the sequence /au/ contracts into a long /oː/ vowel, as in ありがとう from ありがたく.
「ありがとう」の「とう」は、開音の例としてよく挙げられる。
The 'tō' in 'arigatō' is often cited as an example of kai-on.