The courtesy name (字 zì) was a second name given at adulthood in traditional Chinese elite culture. It was used in formal address and by peers; the given name (名 míng) was reserved for family, elders, or self-reference.
Usage
Calling someone by their courtesy name showed respect and recognised their social standing. In the Records of the Three Kingdoms and other sources, figures are routinely referred to by both names: Cao Cao (Mengde 孟德), Liu Bei (Xuande 玄德), Sun Quan (Zhongmou 仲謀). Rulers and superiors might be addressed by title or by courtesy name; use of the given name alone could signal intimacy or deliberate disrespect.
In this wiki
Character articles give both the given name and the courtesy name in the identity section, with pinyin and Chinese characters, so readers can follow primary sources and understand how figures were addressed.