Children in those days were classified according to whether they were a child born to the principal wife or concubines. A son of direct descent was born by the legal wife (this was the wife who has been officially married into the family, also known as the principal wife). Being the eldest of the legal wife made him the legitimate heir. They also had higher social status and often received better treatment compared to the sons of non-direct descent born by concubines.
Synonyms:
lawful or common