In
chess, "underpromotion" refers to the act of promoting a
pawn to a piece other than a
queen upon reaching the eighth rank--that is,
promoting to a
knight,
bishop or
rook. One of the most frequent reasons for doing this is to avoid
stalemating the opponent, but there are many other plausible situations where underpromotion occurs. Underpromotion to a knight, for example, can be deadly if the newborn knight immediately
forks the opponent's
king and queen from its promotion square. Underpromotion can even be a psychological
tactic, to emphasise to the opponent that a queen is not needed to defeat him, or (in cases where the promotion square is under attack) to make him unsure whether to capture the promoted piece.
For one of the most famous endgame positions in which underpromotion plays a vital role, see Saavedra position.