[1][2] On'yomi pronunciations are generally classified into go-on, kan-on, tō-on and kan'yō-on, roughly based on when they were borrowed from China.
Additionally, many Chinese syllables, especially those with an entering tone, did not fit the largely consonant-vowel (CV) phonotactics of classical Japanese.
Thus most on'yomi are composed of two morae (beats), the second of which is either a lengthening of the vowel in the first mora (to ei, ō, or ū), the vowel i, or one of the syllables ku, ki, tsu, chi, fu (historically, later merged into ō and ū), or moraic n, chosen for their approximation to the final consonants of Middle Chinese.
The go-on readings are especially common in Buddhist terminology such as gokuraku (極楽, paradise), as well as in some of the earliest loans, such as the Sino-Japanese numbers.
The tō-on readings occur in some later words, such as isu (椅子, chair), futon (布団, mattress), and andon (行灯, a kind of paper lantern).