Polygamy

Dt 21:15-17

If a man has two wives, one loved and the other unloved, and if both the loved and the unloved bear him sons, but the firstborn is the son of the unloved wife: when he comes to bequeath his property to his sons he may not consider as his firstborn the son of the wife he loves, in preference to the son of the wife he does not love, the firstborn. On the contrary, he shall recognize as his firstborn the son of the unloved wife, giving him a double share of whatever he happens to own, since he is the first fruits of his manhood, and to him belong the rights of the firstborn.

Wikipedia, "Keturah."

Keturah is referred to at different times as either Abraham's wife in Genesis 25:1 or Abraham's concubine in 1 Chronicles 1:32. While Abraham left everything to Isaac, he made grants to his sons by his concubine during his lifetime, and sent them east away from Isaac. He died at the age of 175. Keturah's six sons represent Arabian tribes south and east of Canaan.