Adoption by arms.
An ancient custom of giving arms to a person of merit, which laid him under the obligation of being your champion and defender.
Adoption by baptism.
Being godfather or godmother to a child. The child by baptism is your godchild.
Adoption by hair.
Cutting off your hair, and giving it to a person in proof that you receive him as your adopted father. Thus Boson, King of Arles, cut off his hair and gave it to Pope John VIII, who adopted him.