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.

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.

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.