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This article uses algebraic notation to describe chess moves.

In chess, a blunder is a very bad move which is quickly recognised as a very bad move by the player who made it, typically before or directly after his opponent has made his reply move.^[1] They are usually caused by some oversight, whether from time pressure (see time control), overconfidence or carelessness. While blunders may seem like a stroke of luck for the opposing player, some chess players give their opponent plenty of opportunities to blunder.

What qualifies as a "blunder" rather than a normal mistake is somewhat subjective. A weak move from a novice player might be explained by the player's lack of skill, while the same move from a master might be called a blunder. In chess annotation, blunders are typically marked with a double question mark, "??", after the move.

One technique formerly recommended to avoid blunders, for example by authors like Simon Webb in Chess for Tigers, was to write down the planned move on the scoresheet, then take one last look before making it. This practice was not uncommon even at the grandmaster level. However, in 2005 the International Chess Federation (FIDE) banned it, instead requiring that the move be made before being written down.^[2] The US Chess Federation also implemented this rule, effective as of January 1, 2007 (a change to rule 15A)^[3].

Contents 1 Grandmaster examples 1.1 Tigran Petrosian vs. David Bronstein 1.2 Alexander Beliavsky v. Leif Erlend Johannessen 1.3 Murray Chandler v. Susan Polgar 1.4 Deep Fritz v. Vladimir Kramnik 2 Complete game scores of the examples 3 See also 4 References

There are instances where strong players, even International Grandmasters make elementary blunders.

The position in the diagram at the right arose in the 1956 Candidates Tournament in Amsterdam. Petrosian, playing White, enjoys a clear advantage with strong knights, active rooks and plenty of mobility while Black's position is congested and hardly able to move. In fact Bronstein, playing Black, has for the last seven moves been making only apparently aimless knight moves, Nc6-d4-c6-d4, and now has played ...Nd4-f5, threatening White's queen, while White has been slowly strengthening his position. White can now easily preserve the positional advantage by a move like 36.Qc7, but overlooking that the queen was en prise, he played 36.Ng5?? and resigned after 36...Nxd6.

This example, from a game played in Linares in 2002, is one of the very rare circumstances where a grandmaster makes the literally worst move on the board, the only one allowing mate on the next move. This is a balanced queen endgame, but Beliavsky playing White is careless. After 69.Kf4?? he had overlooked the response 69...Qb8 checkmate.

This example, from a tournament in Biel in 1987, did not lead to a loss for the blunderer, but an embarrassing draw for the British GM Chandler. Susan Polgar has just played the wily trap 53...Ng8-h6!?, and Chandler realizes that after 54.gxh6+ Kxh6 he will be left with the considerable material advantage of a rook pawn and bishop against a bare king. However, since the bishop is unable to control the queening square h8, Black will draw if she is able to get her king to control h8. But Chandler calculates further, and realizes that it is he who will win control over the h8 square after 55.Kf6, and thereby win the game.^[4]

Therefore Chandler played 54.gxh6+??, but instead of the expected 54...Kxh6 came 54...Kh8! This is in fact almost the same king + bishop + rook pawn v. bare king situation as Chandler had calculated that he would avoid, and the small difference that White has two rook pawns rather than one has no effect on the result. Black controls the h8 square and cannot be chased or squeezed away from it, and so White cannot queen his pawn. After 55.Bd5 Kh7 56.Kf7 Kh8 the players agreed to a draw.

Had Chandler played 54.h4, he would have maintained his winning position.

In November 2006, reigning World Chess Champion Vladimir Kramnik competed in the World Chess Challenge: Man vs. Machine, a six game match against the chess computer Deep Fritz in Bonn, Germany. After the first game had ended in a draw, Kramnik, playing Black, was generally considered in a comfortable position in Game 2, and he thought so himself apparently, as he refused a draw by avoiding a potential threefold repetition on 29...Qa7. Kramnik's troubles began, when he decided to play for a win and pushed his a-pawn, 31...a4. Commentators, including American grandmaster Yasser Seirawan, voiced concerns about Kramnik's intentions and the situation became more uncertain as the game went on with 32.Nxe6 Bxe3+ 33.Kh1 Bxc1 34.Nxf8, turning it into a likely draw.^[5] The game could have ended with 34...Kg8 35.Ng6 Bxb2 36.Qd5+ Kh7 37.Nf8+ Kh8 38.Ng6+.

However Kramnik's next move, 34...Qe3?? (a move which was awarded "???" originally, by Chessbase on a story covering Kramnik's blunder), came as a big surprise and was described as the possible "blunder of the century" by four time Women's World Chess Champion, Susan Polgar, as Kramnik — incredibly — overlooked a mate in one.^[6] Deep Fritz quickly ended the game on the next move with 35.Qh7#, checkmate. Seirawan later called Kramnik's move "a tragedy."

ChessBase described the events as follows, "Kramnik played the move 34...Qe3 calmly, stood up, picked up his cup and was about to leave the stage to go to his rest room. At least one audio commentator also noticed nothing, while Fritz operator Mathias Feist kept glancing from the board to the screen and back, hardly able to believe that he had input the correct move. Fritz was displaying mate in one, and when Mathias executed it on the board Kramnik briefly grasped his forehead, took a seat to sign the score sheet and left for the press conference,"^[7] during which he stated that he had planned the supposedly winning move 34...Qe3 already when playing 29...Qa7, and had rechecked the line after each subsequent move. After exchange of queens Black would win easily with his distant pawn; after 35.Qxb4 Qe2 or 35.Ng6+ Kh7 36.Nf8+ Kg8 black also wins eventually. Chess trainer Alexander Roshal attempted to explain the blunder by saying that the mating pattern of a queen on h7 protected by a knight on f8 is extremely rare and not contained in a grandmaster's automatic repertoire.^[8]

• Tigran Petrosian v. David Bronstein, Amsterdam 1956
• Alexander Beliavsky v. Leif Erlend Johannessen, Linares 2002
• Murray Chandler v. Susan Polgar, Biel 1987
• Deep Fritz v. Vladimir Kramnik, Bonn 2006

• Back rank checkmate