Pronunciation of 九


The dictionary I'm using lists two common pronunciations of 「九」: 「きゅう」 and
「く」. I've usually heard it as 「きゅう」 before, but when saying 「９時」, it always
seems to be 「くじ」. Is there a rule to this?




Today, きゅう is the default (i.e. productive) _on-yomi_ pronunciation of 九 (or
9) for counting most things, and only a small portion of words requires く.

Always:  
**hours** (o'clock) (`9時`, `19時`), **dates** (`19日`, `29日`), **month name**
(`9月`)

Preferred or alternative to きゅう:  
**hours** (duration) (`9時間`, `19時間`...), **years** (`9年(間)`, `2009年`...),
**people** (`9人`, `19人`...), **degree** (`29度`, `39度`...), **bare double-
figure numbers** (`19`, `29`, `1029`...), **-fold, -ple** (`9重`, `19重`...),
followed by **次** (`第9次`, `9次元`...)

Sporadic traditional or euphonic examples (Google it for details):  
`九段`, `九十九里`, `四十九日`, `九分九厘`, `九条`, `九九`, `九曜`, `第九`, `月9` (= 月曜9時) etc.  
(There are far more old words; historically く had been the prevailing
pronunciation until it was replaced because of its homophony with 苦{く}
"anguish".)

