Why is 木の下 pronounced このした instead of きのした?


Does this pronunciation follow a rule of some sort? I couldn't find any
answers online about this example specifically.




There are several websites saying that reading 木 as こ is an example of 転音.

  * 木 alone is always read as き
  * 木 + 陰 is read as こかげ

[This chiebukuro
answer](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q10172573345)
lists such words. It seems much more common for surnames (e.g. 木暮=こぐれ). (NB
the linked answer doesn't say it's a 転音.)

* * *

Personally I have some doubts き→こ is really an instance of 転音. First, I guess
a large majority of native speakers would read 木の下 as きのした. (Not that このした is
wrong, there is a dictionary entry.)

A few other words with 木の..:

  1. 木の実
  2. 木の芽

Of these, 1 would be read mainly as このみ, while きのみ is perfectly acceptable. As
for 2, I think きのめ is more common than このめ. Anyway, these two 木 have both
readings of き and こ, as 木の下.

[This](https://japanknowledge.com/articles/blognihongo/entry.html?entryid=368)
says

>
> 「このめ」の方が「きのめ」よりも古い例が存在することも確かである。『日本国語大辞典』の「このめ」の例は、サンショウの新芽のことではないが、平安時代の右大将藤原道綱の母の日記『蜻蛉日記（かげろうにっき）』（974年頃成立）の`「三月になりぬ。このめすずめがくれになりて（＝3月になった。木の芽が茂ってスズメの姿が隠れるほどになり）」`
> という例が一番古い。

A written form appearing earlier may not prove everything, but it should be
safe to say that こ reading is just as old as き.

Based on these, my opinion is that both き and こ have existed as very old
native Japanese words, and somehow combined with the introduction of the
Chinese character 木. Hence the answer to the question is 'that is just the way
it is'.




There are several websites saying that reading 木 as こ is an example of 転音.

  * 木 alone is always read as き
  * 木 + 陰 is read as こかげ

[This chiebukuro
answer](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q10172573345)
lists such words. It seems much more common for surnames (e.g. 木暮=こぐれ). (NB
the linked answer doesn't say it's a 転音.)

* * *

Personally I have some doubts き→こ is really an instance of 転音. First, I guess
a large majority of native speakers would read 木の下 as きのした. (Not that このした is
wrong, there is a dictionary entry.)

A few other words with 木の..:

  1. 木の実
  2. 木の芽

Of these, 1 would be read mainly as このみ, while きのみ is perfectly acceptable. As
for 2, I think きのめ is more common than このめ. Anyway, these two 木 have both
readings of き and こ, as 木の下.

[This](https://japanknowledge.com/articles/blognihongo/entry.html?entryid=368)
says

>
> 「このめ」の方が「きのめ」よりも古い例が存在することも確かである。『日本国語大辞典』の「このめ」の例は、サンショウの新芽のことではないが、平安時代の右大将藤原道綱の母の日記『蜻蛉日記（かげろうにっき）』（974年頃成立）の`「三月になりぬ。このめすずめがくれになりて（＝3月になった。木の芽が茂ってスズメの姿が隠れるほどになり）」`
> という例が一番古い。

A written form appearing earlier may not prove everything, but it should be
safe to say that こ reading is just as old as き.

Based on these, my opinion is that both き and こ have existed as very old
native Japanese words, and somehow combined with the introduction of the
Chinese character 木. Hence the answer to the question is 'that is just the way
it is'.

