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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?)

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up vote 7 down vote accepted

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).

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Your guess is correct.

The readings that end with -you were borrowed from southern China (possibly through Korea) in the 5th and 6th centuries. They are known as go-on 呉音.

The readings that end with -ei were borrowed from northern China during the Tang dynasty, 7th - 9th centuries. They are known as kan-on 漢音.

You can find a table of go-on/kan-on sound changes here.

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Originally the -you ones ended in -eu, so there's more similarity between the two than is immediately apparent. – Sjiveru Apr 30 '14 at 20:55
Not all of them. In fact, all of the examples in the OP come from -yau. – Zhen Lin May 1 '14 at 0:06

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