Perpetual check

In the game of chess, perpetual check is a situation in which one player can a draw by an unending series of checks. This typically arises when the player who is checking cannot deliver checkmate, and failing to continue the series of checks gives the opponent at least a chance to win. A draw by perpetual check is no longer one of the rules of chess; however, such a situation will eventually result in a draw by either threefold repetition or the fifty-move rule. Players usually agree to a draw long before that, however. .
Perpetual check can also occur in other forms of chess, although the rules relating to it might be different. For example, giving perpetual check is not allowed in shogi and xiangqi, where doing so leads to an automatic loss for the giver.
Examples.
In this diagram, Black is ahead a rook, a bishop, and a pawn, which would normally be a decisive advantage. But White, to move, can draw by perpetual check:
The same position will soon repeat for the third time and White can claim a draw by threefold repetition; or the players will agree to a draw. 
Unzicker versus Averbakh.
In the diagram, from Unzicker–Averbakh, Stockholm Interzonal 1952, Black (on move) would soon be forced to give up one of his rooks for White's c-pawn (to prevent it from promoting or to capture the promoted queen after promotion). He can, however, exploit the weakness of White's pawn structure with 
Threatening 3...Qh2.
Salvaging a draw by threefold repetition with checks by moving the queen alternatively to f2 and h4.
Hamppe versus Meitner.
In a classic game Carl Hamppe–Philipp Meitner, Vienna 1872, following a series of sacrifices Black forced the game to the position in the diagram, where he drew by a perpetual check: 
If 17.Kxb7 Kd7 18.Qg4+ Kd6 followed by ...Rhb8#.
If 18.Ka4? Bc4 and 19...b5#.
Leko versus Kramnik.
In the game Peter Leko–Vladimir Kramnik, Corus 2008, Black was able to obtain a draw because of perpetual check:
If 28. Kd2? Rd8+ 29. Ke2 Qe7+
Fischer versus Tal.
A perpetual check saved a draw for Mikhail Tal in the game Bobby Fischer–Tal, Leipzig 1960, played in the 14th Chess Olympiad, while Tal was World Champion. In this position Black played 
and the game was drawn . (After 22.Kh1, then 22...Qf3+ 23.Kg1 Qg4+ forces perpetual check.) 
History.
The "Oxford Encyclopedia of Chess Games, Volume 1 (1485–1866)" includes all recorded games played up to 1800 . The earliest example of perpetual check contained in it is a game played by two unknown players in 1750:
The next examples of perpetual check in the book are two games, both ending in perpetual check, played in 1788 between Bowdler and Philidor, with Philidor giving odds of pawn and move .
A draw by perpetual check used to be in the rules of chess , . Howard Staunton gave it as one of six ways to draw a game in "The Chess-Player's Handbook" . It has since been removed because perpetual check will eventually allow a draw claim by either threefold repetition or the fifty-move rule. If a player demonstrates intent to perform perpetual check, the players usually agree to a draw .
References.
Bibliography