A theory has been developed about this distinction between A-type and B-type, saying the distinction stemed from sounds; in ancient times, there were eight vowels, which were five vowels of アイウエオ (a, i, u, e, o) with each of イエオ (i, e, o) having two types, A-type and B-type.
こうした甲乙の区別は、上代においては母音がアイウエオの5音の他にイエオのみ甲乙の2種類に分かれ、8母音であったことから生じたという音韻による区別の説が立てられた。