In chess, a pure mate is a checkmate position such that the mated king is attacked exactly once, and prevented from moving to any of the adjacent squares in its for exactly one reason per square. Each of the squares in the mated king's field is attacked or "guarded" by one—and only one—attacking unit, or else a square which is not attacked is occupied by a friendly unit, a unit of the same color as the mated king. Some authors allow that special situations involving double check or pins may also be considered as pure mate.
Pure mates are of interest to chess problem composers for their aesthetic value. In real gameplay, their occurrence is incidental. Nevertheless, several famous games have concluded with a pure mate, including the Immortal Game and the Evergreen Game, both won by Adolf Anderssen; the Peruvian Immortal; and the Game of the Century, an early won by Bobby Fischer. Siegbert Tarrasch was fond of pure mates, often pointing them out when they occurred.
Pure mate is one of a few terms used by composers to describe the properties of a checkmate position; related concepts include economical mate, model mate, and ideal mate. An economical mate is a position such that all of the attacker's pieces contribute to the checkmate, with the (optional) exception of the king and the pawns. The model mate and the ideal mate are both stronger forms of pure mate. When a checkmate is both "pure and economical", it is said to be a model mate. In practice, this means that for each of the squares in the king's field, there is exactly one reason why the mated king cannot be moved to that square to escape check. Some authors omit the detail that the mated king is attacked only once. In his Dictionary of Modern Chess, Byrne J. Horton provided the following definition:
Horton's definition assumes a historical norm in chess problems, in which White is the attacking and mating side, while Black is the mated and losing side. However, the concept of a pure mate is also applicable to positions in which Black is the mating side.
In the Oxford Companion to Chess, David Hooper and Kenneth Whyld gave a more complex definition, allowing for exceptions in the cases of pins and double check:
Examples
Steinkühler vs. Blackburne, 1863
After providing their own definition, Hooper and Whyld cited a won by Joseph Henry Blackburne against Alexander Steinkühler as an example of pure mate with Black as the mating side. Playing as Black and beginning with Black's 19th move, Blackburne initiated an attack on White's consisting of three consecutive checks (19...Qxf2+ 20.Kh1 Qg1+ 21.Rxg1 Nf2+ 22.Kg2) such that after each check, White had exactly one legal response, involving a queen sacrifice. Black then played his only available check (and mate) 22...Bh3, a pure mate in which Black's knight, light-squared bishop and king's rook with each other to give mate.
In the final position, the White king occupied a square away from the board's edge, such that its field consisted of the maximum eight squares. The three squares g1, h2 and g3 were blocked by friendly pieces (referred to by Hooper and Whyld as self-blocks), none of which were also attacked by opposing units. h1 and h3 were attacked (or guarded) by the knight, and f3 and f2 were attacked (or guarded) by the king's rook. The square f1 is not attacked by the rook (the knight limits its line of attack); however, it is by the bishop in that although the bishop may be said not to attack the square in the given position, the king is nevertheless prevented from fleeing along the bishop's line of attack, as it would remain in check. For this reason, some authors prefer to use the word "guarded" rather than "attacked" to describe such limitations on the king's movement.
The "Game of the Century"
In 1956, Bobby Fischer, a 13-year-old boy and future world champion, won a famous game against the master player Donald Byrne, establishing himself as a skilled player. Following an early queen sacrifice, Fischer coordinated his pieces to win a rook, two bishops, and a pawn for the sacrificed queen, thus gaining a winning material advantage.
In professional chess, when a game is clearly lost, it is customary for the losing player to resign rather than play until checkmate, both out of respect for one's opponent and also to avoid the tedium of forcing the opponent to play until checkmate, which the winning player may view as poor sportsmanship. However, Byrne is supposed to have conferred with some of his fellow players, deciding to play until checkmate for the same reason that most players resign ahead of time: as a sign of respect for Fischer's skill and also to allow the young Fischer the satisfaction of mate. The final result was a pure mate, with a black rook, bishop and knight trapping the white king against the edge of the board.
The "Immortal Game"
The Immortal Game was a casual game played by Adolf Anderssen and Lionel Kieseritzky in 1851. Anderssen allowed a double rook sacrifice in order to develop an attack using his remaining . Hooper, Whyld and Kasparov report that Kieseritzky resigned the game following 20.Ke2. However, the continuation 20...Na6 21.Nxg7+ Kd8 22.Qf6+ Nxf6 23.Be7# is commonly reported in treatments of the game. If played, this continuation would have resulted in a model mate. Kasparov described the continuation as being both "pure" and "economic", two adjectives which, taken together, are synonymous with a model mate.