1989 年 1989 巻 96 号 p. 23-42
Among the five vowels in Modern Japanese, the vowel u is a highcentral unrounded sound [w]. Accordingly, the vowel triangle of Modern Japanese is somewhat anomalous, lacking a rounded high-back vowel [u]. How did this sort of vowel system form historically?
A reconstruction of eight vowels in Old Japanese, the central dialect of the Nara period (710-84), is commonly accepted. Although interpreted otherwise by some, the so-called ko-otsu distinctions must have been due to the vocalic oppositions, i≠ï, e≠ë, and o≠ö. The vowel diagrams of Old Japanese drawn by scholars adopting the eight-vowel theory are, however, in conflict with what some consider to be phonetic universals. In particular, there are only three vowels (i, i, and u) at the high level but four (e, ë, ö, and o) at the mid.
There is no problem with assuming that the sound values of (C) a, (C) i, (C) u, (C) e, and # o were identical with those of the corresponding vowels in Modern Japanese. The problem reduces to determining the sound values of Cï, Cë, Co, and Cö. Judging from the positions of the Chinese Characters, used as man' yogana, in the Ancient Chinese rhyme tables, one may infer that -ö(following a consonant) was phonetically identical with # o (not following a consonant), which was [o], and that the sound value of -o was [u], a high-back vowel. Thus, o with no preceding consonant should be described as ö. Likewise, ï was very likely high-central and ë mid-central. Drawing a diagram based on the inferred sound values, i, ï, u, and o form a high vowel series, front to back; e, ë, and ö form a mid vowel series, front to back; and a is a low-central vowel, just as in the vowel triangles of Altaic languages.
Internal reconstruction on the basis of the verbal stems of Old Japanese indicates that all eight vowels could be coupled with all consonants, including zero, in Archaic (pre-Nara) Japanese. The eighth century was the close of a transitional era, when the ko-otsu distinction remained only in a few environments. At the outset of the Heian period (794-1192), the phonetic changes ï>i, ë>e, and o>ö took place in all environments. The remaining five vowels have continued into the modern period, forming the “anomalous” vowel system of Modern Japanese.