Why do some numbers have two readings?

Question

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?

Answer 1

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. 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】, 七五三【しちごさん】, 九九【くく】
• In さんし, ろくしち, etc. (see: What is the correct expression of 10/20, 20/30, 30/40 etc?)
• 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?)
• As mnemonics to remember numbers (see: Rules for slang of Japanese numbers)
• Old speakers of some dialects may say しじゅう/しちじゅう/くじゅう.