Those syllables marked in gray are known as Jōdai Tokushu Kanazukai.
The two groups merged together by the 9th century. It predates the development of kana, and the phonetic difference is unclear. Therefore an ad-hoc transcription system is employed.
Syllables written with subscript 1 are known as type and those with subscript 2 as type .
There are several competing transcription systems. One popular system places a dieresis above the vowel: ï, ë, ö. This typically represents i2, e2, and o2, and assumes that unmarked i, e, and o are i1, e1, and o1. It does not necessarily have anything to do with pronunciation. There are several problems with this system.
Another system uses superscripts instead of subscripts.
There are many hypotheses to explain the distinction. However, it is not clear whether the distinction applied to the consonant, vowel, or something else. There is no general academic agreement. See Old Japanese for more information.
A word is consistently, without exception, written with syllables from a specific group. For example, /kami1/ "above" and /kami2/ "god". While both words consists of an /m/ and an /i/, mi1 can not substitute for mi2 or vice versa. This strict distinction exists for all of the syllables marked in gray.
This usage is also found in the verb morphology. The conjugation is as follows:
|Verb Class|| Irrealis|
The verb /sak-/ "bloom" has conjugation class. Thus, its conjugation is as follows:
|Verb Class|| Irrealis|
It should be noted that before the Jōdai Tokushu Kanazukai discovery, it was thought that and shared the same form: -e. However, after the discovery, it became clear that was -e2 while was -e1.
Also, Jōdai Tokushu Kanazukai has a profound effect on etymology. It was once thought that /kami/ "above" and /kami/ "god" shared the same etymology, a god being an entity high above. However, after the discovery, it is known that "above" is /kami1/ while "god" is /kami2/. Thus, they are distinct words.
Following is a chart listing syllable and man'yōgana correspondences.
The distinction between /mo1/ and /mo2/ is only made in the oldest text: Kojiki. After that, they merged into /mo/.
In later texts, confusion between type A and B can be seen. Nearly all of the A/B distinctions had vanished by the Classical Japanese period. As seen in early Heian Period texts such as Kogo Shūi, the final syllables to be distinguished were /ko1, go1/ and /ko2, go2/. After the merger, CV1 and CV2 became CV.