Perpetual check is a tactical resource in which a player delivers an unending series of checks that the opponent’s king cannot escape, thereby forcing a draw by repetition.
The mechanism is straightforward: the player who is losing — whether behind in material or facing a decisive threat — finds a sequence of checks that continually attacks the enemy king, with no square available for the king to escape. When the same position occurs three times with the same player to move and the same options, either player may claim a draw under FIDE rules. Note that the fifty-move rule (no capture or pawn move in fifty moves) is a separate drawing mechanism and does not apply here. As a practical example, imagine White has a queen and a knight while Black has an extra queen: if White can bounce the queen between g7 and h6, keeping the black king in perpetual check with no escape, the game ends in a draw.
In practice, perpetual check is one of the most important saving resources every player must know. When your position is under pressure, actively look for whether your queen — or another piece — can reach the enemy king and maintain a relentless series of checks. Often just two or three alternating squares are enough to force the repetition. Conversely, when you hold a material advantage, make sure your king always has a flight square to avoid being caught in an unwanted perpetual check.