Promotion in chess is a rule that requires a pawn that reaches its eighth rank to be immediately replaced by the player's choice of a queen, knight, rook, or bishop of the same color.[1] The new piece replaces the pawn, as part of the same move. The choice of new piece is not limited to pieces previously captured, thus promotion can result in a player owning, for example, two or more queens despite starting the game with one.[2] Pawn promotion, or the threat of it, often decides the result in an endgame. Since the queen is the most powerful piece, the vast majority of promotions are to a queen. Promotion to a queen is also called queening; promotion to any other piece is referred to as 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).
https://en.wikipedia.org/wiki/Promotion_(chess)