Is there a purpose with different readings for the same jukugo kanji?


So I just learned 下唇 and 上唇 and they could be read with both their onyomi and kunyomi readings (かしん/したくちびる and じょうしん/うわくちびる respectively).

Is there a reason that that not only these words but many other words can be said with both on/on reading and kun/kun reading? Is one the “more scientific” term (like heart is also cardiac) or something else? Just trying to figure out if one way of reading should be said in certain situations over the other.



You would need to take it on a case-by-case basis and do the research for any given word.

In the case of 下唇, かしん has more definitions, which do happen to be scientific, than したくちびる. The kun’yomi is the ordinary word that people would actually use when talking about their lower lip in conversation.


But you can’t just assume that will be the case. With a compound like 白鳥, はくちょう is the common everyday word for a swan, while しらとり is a poetic or literary word for a variety of white birds. So the on’yomi one is the one people would expect in conversation.

You just have to check.

And sometimes one or the other will be in the dictionary, but effectively unused in modern Japanese.

Like 事業 is usually read じぎょう, a very common word for “enterprise” or “business venture”. But the kun’yomi, ことわざ, meaning “deed” or “action” is archaic and essentially unknown to modern natives.

If you said ことわざ, they would think of 諺 (proverb).


Or in colloquial terms, we could say that it is because Japanese people wanted to torment foreigners if they dared learn their language ._.

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“So soon he profits in divinity, The fruitful plot of scholarism grac’d, That shortly he was grac’d with doctor’s name, Excelling all whose sweet delight disputes In heavenly matters of theology; Till swoln with cunning, of a self-conceit, His waxen wings did mount above his reach, And, melting, heavens conspir’d his overthrow” --Dr Faustus


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