It is known that among the 50-character syllabary used today, 14 sounds were written in two types of Manyo-gana, namely A-type and B-type, in ancient Japan before the Nara period, and this distinction of the types was strictly maintained; these 14 sounds were "キ, ヒ, ミ" (ki, hi, mi) with the vowel of イ (i), "ケ, へ, メ" (ke, he, me) with the vowel of エ (e), "コ, ソ, ト, ノ, (モ), ヨ, ロ" (ko, so, to, no, [mo], yo, ro) with the vowel of オ (o), and the vowel "エ" (e).
現代日本語の50音のうち、イ段のキ・ヒ・ミ、エ段のケ・へ・メ、オ段のコ・ソ・ト・ノ・（モ）・ヨ・ロ及びエの14音について奈良時代以前の上代には甲類と乙類の万葉仮名の書き分けが見られ、両者は厳格に区別されていたことがわかっている。