In 1981 Hiromichi MORI, in his 'The Northern Dialect of the Tang Dynasty and the Phonetic Value of the Vowels in Old Japanese', published a theory of seven vowels, recognizing the distinction between the A- and B-types other than those with the vowel of エ (e), and tried to presume concrete sounds using the northern dialect of the Tang dynasty and phonetically arranged dictionary of ancient Chinese characters.
1981年には森博達が「唐代北方音と上代日本語の母音評価」でエ段以外の甲乙に差があることを認める7母音説を発表するとともに、唐代北方音と切韻を利用した具体的音価の推定を試みた。