This book examines how the value of pieces changes in the course of a game and how masters use this knowledge to decide which pieces to exchange - and when. It investigates why the traditional 'chart of relative values' or computer analysis so often fails to explain why certain trades and sacrifices are good and others are bad. The book focuses on typical decisions a player has to make - for example, whether to trade two minor pieces for a rook and pawn, or the queen for two rooks.