Page "Emperor of Japan" Paragraph 34

Traditionally, the Japanese considered it disrespectful to call any person by his given name, and more so for a person of noble rank.

This convention is more relaxed in modern age and now it is acceptable among friends to use the given name, but use of the family name is still common.

In the case of the imperial family, it is still considered inappropriate to use the given name.

Since Emperor Meiji, it has been customary to have one era per emperor and to rename each emperor after his death using the name of the era over which he presided, plus the word Tennō.

Prior to Emperor Meiji, the names of the eras were changed more frequently, and the posthumous names of the emperors were chosen in a different manner.