Vowel transformation in compound Japanese words: 母音交代 (boin koutai)

By | November 29, 2021

Compound words are very common in the Japanese language, and these can be especially nice for learners since it allows guessing the meaning and pronunciation of a word you have seen for the first time (assuming you are familiar with the component words). For example, let’s take this word:

  • 売り物 (urimono)

If we know that “uri” is a form of the word “uru” (meaning “to sell”) and “mono” means “physical object” (among other things), we can make a guess that “urimono” means “an article that is for sale”. Let’s look at another simple compound word:

  • 下ろし金 (oroshigane)

This one is a bit trickier since “oroshi” comes from “orosu” that has a few meanings (“to go take down”, “to grate”, “to withdraw”), and “gane” comes from “kane” that means things such as “money” or “metal”. But if you hear this word being used in the context of cooking you might guess that it means “grater” (for things like daikon radishes, etc.)

However, as you start to become exposed to many Japanese compound words you’ll notice a few unexpected things. One is that often the first letter of the second word will have a dakuten (the two little ticks that looks like this -> ゛) added to it if it doesn’t already. In the last word, かね (kane)  goes to がね (gane). If you try to pronounce both “orishikane” and “orishigane”, you’ll notice the latter rolls off the tongue a bit easier, which is probably why this linguistic phenomenon occurs for many words.

But I would like to focus on another transformation that occurs commonly with compound words, this time related to vowels. Let’s look at word for “rain cloud”, which is of course composed of “rain” (雨, ame) and “cloud” (雲, kumo).

  • 雨雲

How do you think this is pronounced? Your first guess might be or “amekumo” or “amegumo”, but in fact it is actually “amagumo”. Notice that the “me” sound changed to “ma”, which we can view as the “e” vowel being transformed to an “a”. Like with the dakuten transformation, this clearly seems to be easier to pronounce, probably because the mouth is already set in place for the “a” at the beginning of the word. There are a bunch of other rain-related words that have this transformation (雨宿り [amayadori], 雨水 [amamizu], etc.) though there are some words that do not follow the pattern (雨男 [ameotoko], 飴玉 [amedama]).

This “e”=>”a” transformation also occurs for other words, like 酒 (“sake”), for example 酒屋 (bar) which is “sakaya” (not “sakeya”).

There are some other vowel transformations, such as “i”=>”o” (木立 [kodachi] = “grove”) and “i”=>”e” (景色 [keshiki] = “scenery”). Also of note is 上 (“ue” meaning “up” or “above”), which often changes to “uwa” when in compound words (上顎, “uwaago”).

By the way, this phenomenon is called 母音交代 (boin koutai), which is literally “vowel change”, although there is also a linguistic term for this in English: “apophony”. Besides the aspect of whether such transformations make words easier to pronounce (which is perhaps a bit subjective), this change can be viewed as a way to provide extra information to the listener/reader that there is a compound word coming as opposed to two separate words, potentially reducing the amount of thought required to parse and understand a sentence. This can be likened to verb conjugation in Japanese, for example 行く (iku) changes to 行けば (ikeba), 行こう (ikou), 行きます (ikimasu), and 行かない (ikanai). Each change of vowel signifies a different meaning.

Another good thing to know about the vowel changes is that they generally occur more common with the Japanese readings (kun-yomi) of the kanji, and very rarely (if never) of the Chinese readings (on-yomi). For example, the on-yomi of 酒 (sake) is “shu” and generally the vowel there never changes in compounds (ex: 酒宴 = “shuen”)

Here is a page that gives you more explanation and examples of these (in Japanese). Below I have prepared a mini quiz so you test your knowledge of a few words with vowel changes.

Mini Quiz

What is the correct way to pronounce each of these compound words? (See answers below)

  1. 上着
  2. 吐き気
  3. 木の葉
  4. 胸元




  1. uwagi
  2. hakike
  3. konoha
  4. munamoto
  5. sakazuki 
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