But the king spared Mephibosheth, the son of Jonathan, the son
As they did not name particular persons, only required seven sons, it was at the option of the king what sons to deliver to them, and therefore kept back Mephibosheth, who is thus described, to distinguish him from a son of Saul's of the same name, after mentioned:
because of the Lord's oath that [was] between them, between David and
Jonathan the son of Saul;
not merely or only out of affection to Mephibosheth, but because of the oath, that he might not be guilty of the same crime Saul was in slaying the Gibeonites.