mukoyōshi

A is an adult man who is adopted into a Japanese family as a daughter's husband, and who takes the family's surname.

Generally in Japan, a woman takes her husband's name and is adopted into his family. When a family, especially one with a well established business, has no male heir but has an unwed daughter of a suitable age, she will marry the mukoyōshi, a man chosen especially for his ability to run the family business. If there is no daughter, the candidate can take a bride from outside his adopted family – . This is done to preserve the business and name of the family when there is no suitable male heir, since traditionally businesses are inherited by the oldest male heir through primogeniture. Mukoyōshi is also practiced if there is no capable male heir to run the family business.