77775
Generally, when we read or count, 4, 7 and 9 are read only as "よん", "なな",
"きゅう", seperately. However, in some cases, 4 is read as "よ" rather than "よん",
7 is read as "しち" rather than "なな", and 9 is read as "く" rather than "きゅう".
Therefore, I would like to ask some question.

i) When is 4 read as "よ", "よん", or both?

ii) When is 7 read as "しち", "なな", or both?

iii) When is 9 read as "く", "きゅう", or both?

iv) Why do we not say "しじゅう", "しちじゅう", and "くじゅう" when counting 1 to 100?
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2020-06-02T03:10:13.963
77772
2020-06-02T15:02:48.243
2020-06-02T15:02:48.243
33435
39179
question
pronunciation
Why do some numbers have two readings?
When you say "four" or "nine" on its own, it's almost always よん and きゅう in
modern Japanese. しち is somehow relatively common as a standalone word, too,
but you can always stick to なな. (いち, に, さん and し are on-readings but よん is a
kun-reading. However, よん took the place of し for the most part, because し was
aurally less distinctive and [し is associated to
death](https://www.tofugu.com/japan/number-four-superstition/). なな are
preferred especially in math or financial contexts because しち sounds a bit
similar to いち.).

However, し, しち and く are used in the following contexts.

  * In month names: 四月【しがつ】, 七月【しちがつ】, 九月【くがつ】 (なながつ is uncommon but acceptable. よんがつ and きゅうがつ sound very strange.)
  * In 七時【しちじ】 and 九時【くじ】 (4 o'clock is よじ, not しじ. ななじ is also acceptable. きゅうじ is always wrong.)
  * In fixed (usu. on-on) compounds: [四方【しほう】](https://jisho.org/word/%E5%9B%9B%E6%96%B9), [七五三【しちごさん】](https://en.wikipedia.org/wiki/Shichi-Go-San), [九九【くく】](https://jisho.org/word/%E4%B9%9D%E4%B9%9D)
  * In さんし, ろくしち, etc. (see: [What is the correct expression of 10/20, 20/30, 30/40 etc?](https://japanese.stackexchange.com/q/68582/5010))
  * To describe an age without 歳/才: 十九【じゅうく】の春, 二十四【にじゅうし】で嫁に行く, 四十【しじゅう】からの健康法 (I don't know why, but this is probably a poetic/literary custom? Usually you can say 二十四歳【にじゅうよんさい】, etc.)
  * When you count **up** only using on-yomi (see: [How do you do a countdown?](https://japanese.stackexchange.com/q/2364/5010))
  * As mnemonics to remember numbers (see: [Rules for slang of Japanese numbers](https://japanese.stackexchange.com/q/853/5010))
  * Old speakers of some dialects may say しじゅう/しちじゅう/くじゅう.
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2020-06-02T05:37:47.217
77775
2020-06-02T05:37:47.217
5010
77772
answer