Why do some numbers have two readings?


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?




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.




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.

