Era names indicate the various reasons for their adoption. For instance, the nengō , during the Nara period, was declared due to the discovery of copper deposits in Chichibu. Most nengō are composed of two kanji, except for a short time during the Nara period when four-kanji names were sometimes adopted to follow the Chinese trend. , , and are some famous nengō names that use four characters. Since the Heian period, Confucian thoughts and ideas have been reflected in era names, such as , and . Although there currently exist a total of 248 Japanese era names, only 73 kanji have been used in composing them. Out of these 73 kanji, 31 of them have been used only once, while the rest have been used repeatedly in different combinations.