A passed pawn must be blockaded, so as to have its power restrained as much as possible. The minor pieces (knight or bishop) are ideal for this purpose, as they can rarely be forced to retreat by enemy action. On the other hand, the major pieces (queen and rook) find it difficult to achieve a stable blockade as is easy to harass them, while one must also consider that, for such valuable pieces, dealing with a mere pawn cannot be an efficient form of employment. Taking the above into account, it becomes clear that the side with the passed pawn should seek to exchange minor pieces and retain the major ones; the opposite applies to the defending side.