The question is very simple: how many kanji there are that have a 'unique' ON-reading, not shared by any other kanji? Or, as it is probably a too general question given the variety of kanji: how many one-kanji ON-readings exist in the Joyo list? Do any of those readings gain additional kanji if we start to consider Jinmeiyo-kanji / non-Joyo-kanji / non-standard readings? Tricky readings like [頁]{ページ} or [糎]{センチメートル} are excluded.

Example, which I know: the チャ reading is only with 茶 while we are in Joyo, but if we extend our search, there are at least 茗 and 楪 which share the same.

  • Interesting question, I wonder if the scope of the question might get to be a little broad (I'm not an expert on kanji). Are you digging for something specific?
    – ajsmart
    May 24 '18 at 17:31
  • 「チャ」cannot be an on’yomi for「茗」or「楪」- that is not their Chinese-derived pronunciation. Those two may have been used as synonyms for「茶」, so this is a case where the characters have a kun’yomi (「ちゃ」) which is from the on’yomi of another character.
    – dROOOze
    May 25 '18 at 0:18
  • @droooze 楪(ちゃ) is a true Chinese-derived reading attested in the loan 楪子(ちゃつ). It is a late borrowing, regarded as 唐宋音. (ちゃ is not a possible 呉音 or 漢音 reading.)
    – muhmuhten
    Nov 26 '19 at 4:02
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    @muhmuhten that's interesting - I currently cannot find a reference or reconstruction which suggests that ちゃ can be derived from a historical reading of 楪. "楪子" is also not a Chinese word, although it might have been derived from the identically pronounced 碟子. But 碟, 楪, and 蝶 are supposed to sound the same or identical, so I don't know where ちゃ comes from.
    – dROOOze
    Nov 26 '19 at 10:07
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    @muhmuhten The initial ch part of チヤ is fine. I initially took issue with the final a part (as opposed to the おう of, say, 蝶), but upon closer inspection of 蝶 in the Minnan and Hakka topolects, this is also believable. Since 唐音 are not systematic, I would regard チャ as an imitation of something that sounded like "diap" in a southern Chinese topolect. Thank you for pointing this out!
    – dROOOze
    Nov 27 '19 at 7:37

According to a list in this answer, there are 65 joyo-kanji which have a unique on-reading (in the sense of "has a reading not shared by other kanji"). They include 茶. Since even easy joyo-kanji have some nonstandard/rare readings which are not covered by all dictionaries, the number should vary depending on the method. For example, the list says 条 has a unique on-reading デキ, but I don't know when 条 can be read like that. Also note that the list seems a bit old. The list says ボッ is unique to 坊, but it's no longer true since 勃, a kanji recently included in the joyo list, can be read as ボッ, too. 鬱 (ウツ) is now a joyo kanji, and probably has a unique on-reading.

  • The list says 条 has a unique on-reading デキ, but I don't know when 条 can be read like that This is a [反切]{はんせつ} reconstruction of the character「滌」(徒歷切, i.e. imperfectly reconstructed due to sound shifts [徒]{ト} + [歷]{レキ} = テキ).「条」(or rather, Kyūjitai「條」) was substituted for the character「滌」in the Rites of Zhou - i.e. obscure, and you can safely discard the reading デキ as having almost no practical value.
    – dROOOze
    May 26 '18 at 1:43

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