Underpromotion

During promotion, a player may choose a queen, rook, bishop, or knight. Choosing any of the three pieces other than a queen is called underpromotion . It is a complete and intentional legal move, not a partial promotion.

A queen controls more squares than the other choices, but more control is not always useful. A knight can give check, a direct attack on the king that requires a reply, or create a fork, a simultaneous attack on two or more targets. A rook or bishop may leave an escape square open that a queen would cover.

Knight with exclusive geometry

Knight promotion with an exclusive check

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Start

g8=N+

The pawn can advance to g8 and choose a promotion piece.

Four legal promotions

The white pawn on g7 can promote on g8. In this geometry, only a knight on g8 will immediately attack the black king on e7.

1. The pawn can advance to g8 and choose a promotion piece.

That narrower control can avoid stalemate, a draw in which the player to move is not in check but has no legal move. Underpromotion can also deliver checkmate, an attack on the king with no legal reply, or gain a tempo by forcing the opponent to respond before carrying out another plan.

Rook that avoids stalemateBishop with exact color-complex control

The rook preserves an escape

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Start

c8=R

The pawn can advance from c7 to c8 and promote.

A queen would control too much

The pawn on c7 can promote on c8. A queen would cover a8, b8, b7, b6, and a6 without attacking the black king on a7.

1. The pawn can advance from c7 to c8 and promote.

The chosen piece becomes active immediately and does not need to have been captured earlier. The correct choice depends on what each piece does from the arrival square, not only on its usual value.