Go-on

The geographic source and route by which go-on reached Japan remain debated. Japanese reference works conventionally describe it as a southern layer relative to kan-on, but discuss both transmission from southern China and transmission through the Korean Peninsula or Baekje, rather than identifying a single source for the entire layer.

Shibatani has noted that go-on readings make up the first of three waves of Chinese loans to the Japanese language, the others being kan-on and tou-sou-on (meaning Tang Song sound), with go-on being mainly associated with Buddhism.

When kan-on readings were introduced, their go-on equivalents did not disappear. Many characters retain readings from both layers, and the reading used depends on the word in which the character occurs. Compounds can also mix the two layers. For example, gengo (言語) combines kan-on gen with go-on go. A comparison of two historical dictionaries found that mixed go-on, kan-on forms among two-character Sino-Japanese words increased from about 15% in the 12th-century Iroha Jiruishō to about 24% in the 1603-1604 Nippo Jisho.

However, some go-on sounds are now lost. Even though monolingual Japanese dictionaries list a complete inventory of go-on for all characters, some were actually reconstructed using the fanqie method or were inferred to be the same as their modern homophones.

These readings were formerly called wa-on (和音; lit. "Japanese sound"). The Nihon Kokugo Daijiten records go-on as a name for the older reading layer from the mid-Heian period, with an early example in Fujiwara no Kintō's Kitayama-shō (compiled c. 1012-1021); before then, wa-on was used. The dictionary notes that the name followed a Chinese contrast between go-on and a pronunciation regarded as correct, but cautions that the label itself encouraged later theories assigning the readings a specifically southern origin.

The correspondences between go-on and kan-on are recurrent tendencies rather than exceptionless rules. Compared with kan-on, go-on more often preserves the distinction between Middle Chinese voiced and voiceless initials. Some Middle Chinese nasal initials are reflected as nasals in go-on but as voiced stops in kan-on.