The phonetic reading of Sino-Japanese characters being used nowadays in Japan mostly originates in Kan-on or Go-on. It is said that Kan-on takes its root in Changan-on of Tang era. A prevailing theory regarding how Go-on was first created is that it had been derived from the phonetic reading system of China’s Southern areas. The standard phonetic reading of Tang was based on the phonetic system of China’s Northern areas, which was changed a lot from the standard phonetic system of the preceding era. But in Southern areas of China, the standard phonetic system of the preceding era still remained in use at that time, which people of Tang called Go-on. Japanese diplomats or monks dispatched to Tang familiarized themselves with the standard phonetic system of the times in Changan, capital of Tang, and took the phonetic reading system with them when coming back to Japan, which they started to call ‘Kan-on,’ meaning something representative of China. On the other hand, the Japanese called the phonetic system of the previous era ‘Go-on,’ just like people of Tang called it that way. In other words, Kan-on and Go-on at that time meant a ‘New phonetic system’ and ‘one from an old time’ respectively. At first, they simply meant new and old phonetic reading systems from China. But, since Kamakura when the phonetic reading of Sino-Japanese characters was totally Japanized, a distinction started to be made between these two; ‘Kan-on’ has come to represent those that strictly followed the phonetic system of Tang, while others long in use from much before were called ‘Go-on’.