pawn en passant must be exercised on the move immediately following the double-step pawn advance, or it is lost for the remainder of the game. The en passant capture is the only capture in chess in which the capturing piece does not replace the captured piece on the same square. Promotion A pawn that advances to its last rank is promoted to a queen, rook, bishop, or knight of the same color. The pawn is replaced by the new piece on the same move. The choice of promotion is not limited to pieces that have been captured; thus, a player could, in theory, have as many as nine queens, ten rooks, ten bishops, or ten knights on the board. Promotion to a queen is also known as queening and to any other piece as underpromotion. Underpromotion is most often to a knight, typically to execute a checkmate or a fork to gain a significant material advantage, among other reasons. Underpromotion to rook or bishop is used to avoid or induce stalemate or for humorous reasons. While some chess sets include an extra queen of each color, most standard sets do not come with additional pieces, so the physical piece used to replace a promoted pawn on the board is usually one that was previously captured. In informal games, when the correct piece is not available, an additional queen is often indicated by inverting a previously captured rook or by placing two pawns on the same square. In tournament games, however, this is not acceptable; in the former case, it may result in the arbiter ruling that the upturned piece is in fact a rook. Strategy Pawn structure The pawn structure, the configuration of pawns on the chessboard, mostly determines the strategic flavor of a game. While other pieces can usually be moved to more favorable positions if they are temporarily badly placed, a poorly positioned pawn is limited in its movement and often cannot be so relocated. Because pawns capture diagonally and can be blocked from moving straight forward, opposing pawns can become locked in diagonal of two or more pawns of each color, where each player controls squares of one color. In the diagram, Black and White have locked their d- and e-pawns. Here, White has a long-term advantage. White will have an easier time than Black in finding good squares for their pieces, particularly with an eye to