Genesis 25:6

Concubines. Agar and Cetura are here called concubines, (though they were lawful wives, and in other places are so called) because they were of an inferior degree: and such in Scripture are usually called concubines. (Challoner) --- The solemnities of marriage were omitted on these occasions, and the children were not entitled to a share in the inheritance. Jacob's two wives consented that all his children, by their handmaids, should be placed on the same footing with their own. (Calmet) --- Abraham contented himself with making suitable presents to the children, whom he had by these secondary wives, reserving the bulk of his property to Isaac, chap. xxiv. 36. He also provided for their establishment himself, that there might be no contest after his departure.