COHABITATION
Concubinage was a lower form of polygamy. The concubine was a woman of lower rank, perhaps a slave or prisoner of war (Gen. 16:3; 22:24; 36:12; Deut. 21:10-11; Judges 5:30; 2 Sam. 5 :13; etc.).
Hagar, p. e.g. she did not have the social position of Sarah (Gal. 4:22, 23), and the children of the concubines, although fully recognized, did not have the same right to inheritance as the children of the main wife (Gal. 4:30; Gen. 25:6).