In ancient China, there were 5 common words to address others respectfully, namely "陛下 (bì xià)", "殿下 (diàn xià)", "阁下 (gé xià)", "麾下 (huī xià)" and "足下 (zú xià)".
Whether you watch a TV play or a movie, as long as an emperor shows up, you will hear the courtiers addressing him "陛下 (bì xià)". Why do they address the emperor "陛下 (bì xià)"? In fact, the character "陛 (bì)" originally referred to the stairs in a royal palace. If a courtier wanted to advise the emperor, he could not call the emperor' name directly. Instead, he should call the attendant at the foot of the stairs, who would inform the emperor. Over time, "陛下 (bì xià)" has become an honorific for an emperor.
"殿下 (diàn xià)" , the same as "陛下 (bì xià)", was originally used to address the emperor respectfully. However, its addressed object varied in different dynasties. Since the Han Dynasty, it had been used to address the crown prince or other princes. Since the Tang Dynasty, only the crown prince, the queen and the empress dowager could be addressed "殿下 (diàn xià)".
"阁下 (gé xià)" is used to address ordinary people politely, often seen in letters. It was originally the title of attendants. When friends or fellows felt embarrassed to call each other's name directly, they would call each other's attendants first. Later, "阁下 (gé xià)" has evolved into an honorific term to address the closest friends or relatives, and it is mostly used on social occasions.
In Chinese, "麾 (huī)" refers to the flag used for commanding an army in ancient China, while "麾下 (huī xià)" is an honorific title for the generals.
In ancient times, "足下 (zú xià)" was commonly used by the inferiors to address the superiors or used between peers. As an honorific title to address others modestly, "足下 (zú xià)" is seldom used nowadays.
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