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I started finally learning kanji's readings not only meanings and words. And
in two sources I use I see that 何's kun'yomi is なん、なに and on' is カ.

I've seen 何 in many words as

なん: 何度、何と、何で

なに: 何人、何か

ど: 何処、何の

何れ (いずれ)、何時(いつ)

  * I know, that ど and い readings can be exceptions.

But never as カ. Are there any words that is it as カ?
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2019-05-22T13:26:19.773
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2019-05-29T11:38:12.227
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question
words kanji onyomi
Are there any words which use 'カ', the on-yomi of 何?
You are correct that there are many more common uses of the 訓読み `なに` or `なん`
for 何 when compared with the words which use the 音読み `カ`.

But here is one example: The word for 'geometry' is:

> 幾何学 (きかがく) -
> [definition](http://kotobank.jp/word/%E5%B9%BE%E4%BD%95%E5%AD%A6-50007)

Although it also seems to appear in words like 如何せん (いかんせん) or 如何 (いかが), I
don't think those are strictly standard usages of the readings but more of a
jukujikun type (reading by meaning). For example, `い` is not a standard
reading of the kanji 如 [(see here for
details)](https://kanji.jitenon.jp/kanjic/1438.html). Therefore I'm not sure
that 如何 qualifies as a usage of the 音読み here.

There are probably more examples - I will check and edit in others if I find
them.
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2019-05-22T13:40:57.840
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2019-05-22T15:14:57.767
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answer
According to kanshudo.com counts, 幾何 and its derivatives are the only words
that can be encountered among the frequent lexicon. However, there is at least
[誰何]{すいか}する “challenging (an unknown person); asking a person's identity”
which is rare but still completely recognizable.

Of course, any (Classical) Chinese word can theoretically be a Japanese one
read like this; from the 四字熟語 list at
<http://www.edrdg.org/projects/yojijukugo.html> I find [無何之郷]{むかのきょう}
[無何有郷]{むかゆうきょう}.
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answer