Endgames can be studied according to the type of pieces that remain on board. For example, king and pawn endgames have only kings and pawns on one or both sides and the task of the stronger side is to promote one of the pawns. Other endings are studied according to the pieces on board other than kings, e.g. rook and pawn versus rook endgame.<ref name=H/W>Hooper D. and Whyld K. 1992. "The Oxford companion to chess". 2nd ed, Oxford University Press.</ref>