ここのつ (kokonotsu) is the kun-yomi when counting things.
キュウ (kyū) and ク (ku) are the on-yomi readings. ク is used in compounds like 九月 (くがつ, kugatsu: September), while キュウ is used in 九州 (きゅうしゅう, kyūshū: Kyūshū).
As part of proper names and set phrases, readings may vary; learn common expressions like 九日 (ここのか, kokonoka: the 9th day of a month).
Examples
Kun-yomiNative Japanese Readings
九つください。
Nine, please.
Uses kun-yomi ここのつ (kokonotsu) when counting things
九つあります。
There are nine.
Uses kun-yomi ここのつ (kokonotsu) when counting things
On-yomiSino-Japanese Readings
九月
September
Uses on-yomi ク (ku) in compound for months
九州
Kyūshū (place name)
Uses on-yomi キュウ (kyū) in compound 九州
九月にいきます。
I will go in September.
Uses on-yomi ク (ku) in compound 九月
ExceptionIrregular Readings
九日
the 9th day of a month
Exception: 九日 reads as ここのか for the ninth day of the month
Quick facts
Radical: 乙 (second)
Strokes: 2
Common compounds: 九月 (くがつ, kugatsu), 九州 (きゅうしゅう, kyūshū: place name)
⚠️ Exceptions:
九日 reads as ここのか when meaning "the 9th day of a month".