Underpromotion

Promotion is a chess rule that a pawn that reaches its eighth rank is immediately changed into the player's choice of a queen, knight, rook, or bishop of the same color. The new piece replaces the pawn on the same square, as part of the same move. The choice of new piece is not limited to pieces that have already been captured; it is legal for promotion to result in a player controlling, for example, two Queens of the same color despite starting the game with one. Promotion is immediate and mandatory for any pawn that reaches its eighth rank, and the pawn may not remain a pawn. Pawn promotion, or the threat of it, often decides the result of a chess endgame.

Since the queen is the most powerful piece, the vast majority of promotions are to a queen. Promotion to a queen is often referred to as queening. A promotion to a piece other than a queen is called underpromotion (Golombek 1977).

If the promoted piece is not physically available, FIDE rules state that the player should stop the game clock and summon the arbiter for the correct piece. Under US Chess Federation rules and in casual play, an upside-down rook may be used to designate a queen (Just & Burg 2003:16–17).

Promotion to a queen is the most common, since the queen is the most powerful piece. Underpromotion (promotion to a piece other than a queen) occurs more often in chess problems than in practical play. In practical play, underpromotions are rare, but not extraordinarily so (see table below). As the most powerful piece, the queen is usually the most desirable, but promotion to a different piece can be advantageous in certain situations. A promotion to knight is occasionally useful, particularly if the knight can give immediate check. A promotion to a rook is occasionally necessary to avoid a draw by immediate stalemate that would occur if the promotion was to a queen. Promotion to a bishop almost never occurs in practical play (about one game in 33,000). (See Underpromotion: Promotion to rook or bishop for examples of underpromotions to rook and bishop made in order to avoid stalemate.)

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