Some kanji were introduced more than once from different parts of China at different times, and so have multiple ''on'yomi'', and often meanings.
Generally, on'yomi are classified into four types:
| Kanji | Meaning | Go'on | Kan'on | Tō'on | Kan'yō'on |
|---|---|---|---|---|---|
| 明 | light | myō | mei | min | * |
| 行 | go | gyō | kō | an | * |
| 極 | extremely | goku | kyoku | * | * |
| 珠 | pearl | * | shu | * | ju, zu |
| 度 | level | do | taku | to | * |
The most common form of readings is the kan'on one. The go'on readings are especially common in Buddhist terminology such as gokuraku (極楽) "paradise". The tō'on readings occur in some words such as isu "chair" or futon.
In Chinese, each character is associated with a single Chinese syllable. Most on'yomi are composed of two moras, the second of which is either a lengthening of the vowel in the first mora, or one of the syllables ku, ki, tsu, chi, or n, chosen for their approximation to the final consonants of Middle Chinese. In fact, palatalized consonants before vowels other than i, as well as syllabic n, were probably added to Japanese to better simulate Chinese; none of these features occur in words of native Japanese origin.
On'yomi primarily occur in multi-kanji compound words (jukugo), many of which are the result of the adoption (along with the kanji themselves) of Chinese words for concepts that either didn't exist in Japanese or could not be articulated as elegantly using native words. Chinese-borrowed terms are often considered to sound more erudite or formal than their native counterparts. Surnames usually use the native kun'yomi.
The kokuji, the kanji invented in Japan, do not normally have on'yomi, but there are exceptions, such as the character 働 'to work', which has the kun'yomi hataraku and the on'yomi dō, and 腺 'gland', which has only the on'yomi sen.