There are many rules in the modern game of chess which arose in order to deal with the possibilities of a game continuing endlessly. Chess clocks were introduced to combat the tendencies of certain players to think for interminable periods of time (see Chess - Time Controls), and in order to limit the length of simple endgames (such as King, Bishop and Knight vs. king), a rule was introduced which declares the game a draw if 50 moves pass without a pawn move or capture.

For similar reasons, a chess game is drawn if exactly the same position recurs three times. This is called a draw by threefold repetition, and the repetition does not necessarily have to occur three times in succession - the third repetition could take place 20 moves after the original two, though that would be highly unusual and might easily pass unnoticed by both players. A threefold repetition can occur by mutual consent, if both players are content with a draw, or it can be forced on one player by another.

This is where Perpetual Check comes in. If a player is losing a game of chess, besides watching out for opportunities to turn the tables and win, they will also be looking out for opportunities to reach an equal position or force a draw, because half a point is, after all, better than no points at all. One of the best ways to force the repetition of a position is through continuous checks which allow the opponent no opportunity to escape. The following example illustrates this:

+----+----+----+----+----+----+----+----+
|    | r  |    |    |    | r  | k  |    |
+----+----+----+----+----+----+----+----+ 
|    |    |    |    |    | p  |    | p  |
+----+----+----+----+----+----+----+----+
|  R |    |    |    |    |    | p  |    | 
+----+----+----+----+----+----+----+----+
|    |    |    |    |    |    | Q  |    | 
+----+----+----+----+----+----+----+----+
|    | q  |    |    |    |    |    |    | 
+----+----+----+----+----+----+----+----+
|    | B  |    |    |    |    |    |    | 
+----+----+----+----+----+----+----+----+
|    |    |    |    |    | P  | P  | P  | 
+----+----+----+----+----+----+----+----+ 
|    |    |    |    |    |    | K  |    |
+----+----+----+----+----+----+----+----+

White is in a lot of trouble here. He is material down (a rook for a minor piece, an imbalance referred to as being down "the exchange"). Black is attacking the bishop on b3 and also threatening instant checkmate on e1 with his queen. It might be time for White to resign, except that there is a saving resource involving a sacrifice:

<b>1.Rxg6+! hxg6 (Black cannot capture the sacrificed rook with the f-pawn because the bishop on b3 pins it to his king. Also, the king cannot escape the check by moving to h8 because then 2.Qf6++ would be checkmate.)
2.Qxg6+ Kh8 (again, the queen cannot be captured because the pawn on f7 is pinned by the bishop)
3.Qh6+ Kg8 (the only move)
4.Qg6+ Kh8
5.Qh6+ Kg8
6.Qg6+ Drawn by perpetual check

+----+----+----+----+----+----+----+----+
|    | r  |    |    |    | r  | k  |    |
+----+----+----+----+----+----+----+----+ 
|    |    |    |    |    | p  |    |    |
+----+----+----+----+----+----+----+----+
|    |    |    |    |    |    | Q  |    | 
+----+----+----+----+----+----+----+----+
|    |    |    |    |    |    |    |    | 
+----+----+----+----+----+----+----+----+
|    | q  |    |    |    |    |    |    | 
+----+----+----+----+----+----+----+----+
|    | B  |    |    |    |    |    |    | 
+----+----+----+----+----+----+----+----+
|    |    |    |    |    | P  | P  | P  | 
+----+----+----+----+----+----+----+----+ 
|    |    |    |    |    |    | K  |    |
+----+----+----+----+----+----+----+----+

It can easily be seen that Black has no option but to accept the repetition of the position, and likewise that White will lose very quickly if he attempts to win or to deviate from the perpetual check. This is a very useful trick to remember in a worse or losing position.

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