A perpetual check is a string of checks the opponent can never escape, so the same position keeps recurring and the game is drawn by threefold repetition.
When you can check the enemy king forever, and it can never find shelter, neither side can make progress. The position repeats, and after the same position occurs three times (or by agreement) the game is a draw.
It’s most often a queen shuttling between two checking squares while the king bounces between two squares of its own. The defender would love to escape but every flight square is met by another check.
Perpetual check is one of the great saving resources: a player who is losing on material can force a draw if the enemy king is exposed enough. When you’re worse, hunting for a perpetual is often your best practical chance.
By repetition. Each check is forced, so the same position keeps returning. Once it occurs three times, either player can claim a draw under the threefold-repetition rule.
When you’re losing, especially down material, and the enemy king is exposed. A perpetual converts a lost game into a draw, so it’s often the best practical try in a bad position.
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