I noticed that in the word 子牛 both "o" and "u" are pronounced while in the word 格子 there is a long "ō" and they are, respectively, written in rōmaji (Hepburn romanization) koushi and kōshi. What I'd like to ask you is if: 1) in Middle Japanese the diphthong "ou" was reduced to "ō" only when the two vowels were part of the same syllable (like in 格子) 2) the reduction happened only in kango and waseigo or it happened also in some yamato kotoba


Is coalescence blocked between syllables?

No, coalescence also occurs between syllables, and even involving vowels from different morphemes. In fact it's not clear whether /ou/ was even a diphthong to being with; it's quite possible that it only ever existed as a hiatus. In modern Tokyo Japanese probably only /ai/, /oi/ and /ui/ are actual diphthongs (every vowel sequence ending in -u needs another syllable for it; they coalesce nonetheless).

Was historical coalescence restricted to kango words?

No, it happened throughout the entire lexicon. Primitive Japanese had no sequential vowels, so yamato-kotoba examples aren't as plentiful as kango; but some vowel sequences did emerge later, and among them, /ou/ did coalesce into /o:/. For example, *saso-pusaso-usasō.

Notice that /au/ also coalesced into /o:/, so that /au/ and /ou/ merged. Diphthongs in /-i/, by contrast, were resistant and survived to this day. So hayauhayō but hayaihayai.

Regarding questions raised in the comment thread:

Is coalescence blocked between morphemes? What about words?


  • Conclusive (終止形) suffix -u: saso-u /saso:/
  • Adjectival suffix -u: samuk-aro-u /samukaro:/
  • 御宇 gyo-u /gyo:/
  • 如雨露 jo-u-ro /Zo:ro/

By contrast:

  • 子牛 ko-usi /kousi/
  • 小唄 ko-uta /kouta/
  • 壇ノ浦 dan-no-ura /daNnoura/
  • 選挙運動 senkyo-undō /seNkjoundo:/
  • 左顧右眄 sako-uben /sakouben/

For the purposes of coalescence, compound words (words made of two free words) count as "different words". So a word like usi or uta won't lose its u-, but a bound suffix might.

Notice that this is consistent with other forms of coalescence in Japanese. So au > ō regardless of whether it was found in native Japanese or Sinitic words, and across morpheme boundaries (haya-u → hayō), but not across word boundaries (matu-ga-ura ­→ matu-ga-ura). Even other sound changes behave similarly; 買ひ → ka-wika-i (cross-morpheme lenition) vs. 朝日 → asa-hi (no cross-word lenition).

What about words like 降雨 or 堂宇?

The final -u doesn't coalesce: /ko:u/, /do:u/. However, the previous /o:/ is already long, so they wouldn't coalesce anyway (/o::/ isn't a legal coda).

Does coalescence behavior differ between yamato-kotoba and kango words?

From the above, it seems like not (except indirectly, insofar as diphthongs are rare in yamato-kotoba and free-word compounds are plentiful).


  • Kubozono, Diphthongs and vowel coalescence (in: The Handbook of Japanese Phonetics and Phonology)
  • Kenkyūsha's Japanese-English dictionary, for pronunciation guides.
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  • Thank you, leoboiko. Yes, /au/ was reduced to /ɔː/ (distinct from /ou/ > /oː/ in Middle Japanese) and then merged with /oː/ (< /ou/) in Modern Japanese (like arigataku > arigatau > arigatɔː > arigatoː). I'm now searching if the /ou/ pronunciation is stable in kango and waseigo like 杜宇, 堂宇, 御宇, 降雨, as it happens in yamato kotoba like 小唄, which is still pronounced /ko.u.ta/. – nino83 May 3 '17 at 10:49
  • I'm not sure. Forvo.com has a 降雨 where I can hear an -u, and Kenkyūsha gives -u forms for 降雨 and 堂宇. However, these are /oou/ sequences; they could hardly become /oːː/. I can't find data on 杜宇 (do people even use the on'yomi of that?), and 御宇 is /gyo:/ in both Kenkyūsha and Suzuki-kun, so I'm guessing the real constraint here might be ou > oo and ouu > oou. – melissa_boiko May 3 '17 at 11:06
  • Probably, but on forvo you can hear 壇ノ浦の戦い pronounced dan'nouranotatakai but, again, "ura" is kun'yomi. I'm wondering if in on'yomi compounds in which the first word ends with an "o" and the second one begins with a "u" the pronunciation is "ou" or "ō". Among jōyō kanji, ,only these ones, 右 宇 羽 雨 鬱 運 雲 , begin with a "u" in their on'yomi pronunciation. – nino83 May 3 '17 at 11:34
  • A bigger list follows: 芋右宇烏羽迂雨欝蔚云運雲怨芸胡佑優有祐于侑傴吁吽嗚塢嫗慍挧暈桙鬱殞熨燠盂禹紆紜繧耘茴薀藝蘊褞謳隕饂鶤齲偊呍噁噢圩圬埡妋姷‌​弙忶惲愪扜抎捥摳杅杇栯橒櫌歍氳洿浟溳漚澐‌​瀀煨熅熰玗琇瑀瑦痀痏盱磒秇竽筠篔縕縜耰耺‌​茡莙菀蒕蓲蕓虶螐觱轀运迶邘鄅鄆鄔鄖醞釪隖‌​雩霣韗韞餫饇骬鰞鵾溫 . However, I found it hard to find cross-kanji /o-u/ sequences in practice. I've reedited my answer to account for things we discussed in this thread. – melissa_boiko May 3 '17 at 13:26
  • 1
    Among your examples, 終止形 -u sound just like /u/ now, though it's tradition to pronounce /o:/ when reciting Classical works. 如雨露 is ateji, it's from Portuguese. 御宇 should be /gyou/ if I remember correctly. – broccoli facemask - cloth May 3 '17 at 21:44

1) in Middle Japanese the diphthong "ou" was reduced to "ō" only when the two vowels were part of the same syllable (like in 格子)

I think what you mean is "the same morpheme", because that's the difference tells 子牛 from 格子. This is a part of four sound shifts have taken place in parallel.

  • //au// → //ɔː// → //oː//
  • //iu// → //yuː//
  • //eu// → //yoː//
  • //ou// → //oː//

This sound shift has had effect mostly inside a morpheme with a few exceptions, but most of those exceptions are eventually reverted in today's language. Some examples of surviving exceptional outcomes are:

  • the volitional suffix: 行かむ //yuka-mu// → 行こう //ikoː//, 出でむ //ide-mu// → //(i)dyoː// (→ leveled to 出よう //deyoː//) etc.
  • -うと/-うど pseudo-suffix series: 素人 //sira-bito// → //siraudo// → //siroːto//, 玄人 //kuro-bito// → //kuroudo// → //kuroːto// etc.
  • 酔ふ //weɸ-u// → //eu// → //yoː// (→ leveled to 酔う //yo-u//)
  • 言ふ //iɸ-u// → //iu// → //yuː// (deemed non-standard now)

In fact, 格子 has never been pronounced like //kousi// despite its modern spelling, because its actual historical development is //kakusi// → //kausi// → //kɔːsi// → //koːsi//. The reason it's spelled こうし is because the Modern Kana Orthography defines that all historical おう, おふ, あう, あふ that pronounced as //oː// should be replaced with おう.

2) the reduction happened only in kango and waseigo or it happened also in some yamato kotoba

This sound change affected all qualified diphthongs regardless of kango or wago (yamato-kotoba), but since such combinations are rare in native Japanese words, the great portion of the effect was seemingly exerted on kango.

  • 塔 (kango) //taɸu// → //tau// → //toː//
  • 尊き (wago) //taɸuto-ki// → //tauto-i// → 尊い //toːto-i//

But some wago apparently showed some degree of resistance.

  • 倒るる //taɸur-uru// → //taur-uru// → 倒れる //taore-ru//
    cf. 放る //ɸaɸur-u// → //ɸaur-u// → //hoːr-u//
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  • Thanks, broccoli forest. Why do the Japanese use the spelling おう in place of おお in words like 格子? Was this practice developed during the Early Modern Japanese period (first half of the 17th century), when /ɔː/ (</au/) merged with /oː/ (</ou/). In other words, I'd expect おお from a modern phonemic orthography or あう from an etymological orthography, not おう, which is neither modern nor etymological. – nino83 May 3 '17 at 12:29
  • @nino83 See my edited answer. – broccoli facemask - cloth May 3 '17 at 12:38
  • Thank you! So this reform was made in 1946, more or less. – nino83 May 3 '17 at 12:46

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