Then Judah said to his daughter-in-law Tamar, “Live as a widow in your father’s house until my son Shelah grows up”; for he thought, “I am afraid lest he also die like his brothers.” So Tamar went and lived in her father’s house.
And Onan knew that the seed would not be his; and it happened that when he went in to his brother’s wife, he wasted it on the ground in order not to give seed to his brother.
“If brothers live together and one of them dies and has no son, then the wife of the one who died shall not be married outside the family to a strange man. Her husband’s brother shall go in to her and take her to himself as wife and perform the duty of a husband’s brother to her.