Naming taboo

A naming taboo is a cultural taboo against speaking or writing the given names of exalted persons, for example in China and within the Chinese cultural sphere. It was enforced by several laws throughout Imperial China, but its cultural and possibly religious origins predate the Qin dynasty. Not respecting the appropriate naming taboos was considered a sign of lacking education and respect, and brought shame both to the offender and the offended person.
Types.
Methods to avoid offense.
There were three ways to avoid using a taboo character:
In history.
Throughout Chinese history, there were emperors whose names contained common characters who would try to alleviate the burden of the populace in practicing name avoidance. For example, Emperor Xuan of Han, whose given name Bingyi (病已) contained two very common characters, changed his name to Xun (詢), a far less common character, with the stated purpose of making it easier for his people to avoid using his name. Similarly, Emperor Taizong of Tang, whose given name Shimin (世民) also contained two very common characters, ordered that name avoidance only required the avoidance of the characters "Shi" and "Min" in direct succession and that it did not require the avoidance of those characters in isolation.
However, Emperor Taizong's son Emperor Gaozong of Tang effectively made this edict ineffective after his death, by requiring the complete avoidance of the characters "Shi" and "Min", necessitating the chancellor Li Shiji to change his name to Li Ji. In later dynasties, princes were frequently given names that contained uncommon characters to make it easier for the public to avoid them, should they become emperor later in life.
During the rule of the Ming Emperor of Han (Liu Zhuang), whose personal name was Zhuang, most people with surname Zhuang (莊) were ordered to change their names to Yan (). 
The custom of naming taboo had a built-in contradiction: without knowing what the emperors' names were, one could hardly be expected to avoid them, thus somehow the emperors' names had to be informally transmitted to the populace to allow them to take cognizance of and thus avoid using said characters. In one famous incident in 435, during the Northern Wei Dynasty, Goguryeo ambassadors made a formal request that the imperial government issue them a document containing the emperors' names so that they could avoid offending the emperor while submitting their king's petition. Emperor Taiwu of Northern Wei agreed and issued them such a document. However, the mechanism of how the regular populace would be able to learn the emperors' names remained generally unclear throughout Chinese history.
This taboo is important to keep in mind when studying ancient historical texts from the cultural sphere, as historical characters and/or locations may be renamed if they happen to share a name with the emperor in power (or previous emperors of the same dynasty) when the text was written. Thus, the study of naming taboos can also help date an ancient text.
In other countries.
Japan was also influenced by the naming taboo. In modern Japan, it concerns only the Emperor of Japan, whom people only refer as Tennō Heika (; his Majesty the Emperor) or Kinjō Heika (; his current Majesty). Historically, it was considered very rude to call someone else's real name, even if it was the lord calling his vassals. Calling someone else's real name was equivalent to picking a fight. Titles or pseudonyms were often used when calling others in place of their real names. However, this does not apply to the Chinese characters in written names.
In Vietnam, the family name Hoàng (黃) was changed to Huỳnh in the South due to the naming taboo of Lord Nguyễn Hoàng's name. Similarly, the family name "Vũ" (武) is known as "Võ" in the South.