-ei/-you alternation in some kanji: what's going on?


There's a reasonable number of kanji that have _on_ -readings like めい・みょう,
へい・ひょう (possibly modulo voicing), etc. For example:

  * 平: 平均【へいきん】 vs. 平等【びょうどう】
  * 兵: 兵士【へいし】 vs. 兵庫県【ひょうごけん】
  * 明: 不明【ふめい】 vs. 明朝【みょうちょう】
  * 京: 京阪【けいはん】 vs. 上京【じょうきょう】
  * 令: 命令【めいれい】 vs. 律令【りつりょう】

Is there some historical reason for this, or is it just a coincidence that all
these kanji have -ei and -you readings? (My guess is that it has something to
do with sound changes in Chinese resulting in borrowings at different times
coming with different _on_ -readings; am I right?)




All of the examples you cite rhyme (modulo tone) in Mandarin: 平 píng, 兵 bīng,
明 míng, 京 jīng, 令 lìng. In fact the first four even rhyme exactly in Middle
Chinese (bjæŋ, pjæŋ, mjæŋ, and kjæŋ, respectively; all level tone), and the
last one is very close (leŋ + falling tone). In go-on, these rimes are
reflected as -yau (or perhaps -yaũ), which become -yō in modern Japanese, and
the corresponding kan-on has -ei.

Incidentally, the reason why 平 has a voiced initial in go-on is because it has
a voiced (= 全濁) initial in Middle Chinese as well. This is reflected in
Mandarin as the rising tone. As a general rule, Middle Chinese voiced initials
become unvoiced in kan-on (and indeed, in most modern varieties of Chinese).




All of the examples you cite rhyme (modulo tone) in Mandarin: 平 píng, 兵 bīng,
明 míng, 京 jīng, 令 lìng. In fact the first four even rhyme exactly in Middle
Chinese (bjæŋ, pjæŋ, mjæŋ, and kjæŋ, respectively; all level tone), and the
last one is very close (leŋ + falling tone). In go-on, these rimes are
reflected as -yau (or perhaps -yaũ), which become -yō in modern Japanese, and
the corresponding kan-on has -ei.

Incidentally, the reason why 平 has a voiced initial in go-on is because it has
a voiced (= 全濁) initial in Middle Chinese as well. This is reflected in
Mandarin as the rising tone. As a general rule, Middle Chinese voiced initials
become unvoiced in kan-on (and indeed, in most modern varieties of Chinese).

