Promotion square

A rank is a horizontal row of eight squares. A promotion square is not one fixed square for the whole board. For White, it is any pawn destination allowed by the rules on rank 8; for Black, it is any allowed destination on the first rank. The pawn may arrive by moving straight into an empty square or by capturing diagonally, as the position allows.

On arrival, the pawn must be replaced as part of the same move. The player chooses a queen, rook, bishop, or knight of the same color. The pawn cannot remain a pawn or become a king. The choice is also independent of earlier captures, so having two or more queens of one color is legal.

Choosing a rook, bishop, or knight instead of a queen is called underpromotion. A queen is usually strongest, but an underpromotion may be necessary because the pieces control squares in different ways. A knight, for example, can attack a target from the destination square that a queen would not attack from there. The new piece takes effect immediately as the result of the promotion.