blunder

In chess, a blunder is a critically bad mistake that severely worsens the player's position by allowing a loss of material, checkmate, or anything similar. It is usually caused by some tactical oversight, whether due to time trouble, overconfidence, or carelessness. Although blunders are most common in beginner games, all human players make them, even at the world championship level. Creating opportunities for the opponent to blunder is an important skill in over-the-board chess.
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.
Especially among amateur and novice players, blunders often occur because of a faulty thought process where players do not consider the opponent's forcing moves. In particular, checks, captures, and threats need to be considered at each move. Neglecting these possibilities leaves a player vulnerable to simple tactical errors.
One technique formerly recommended to avoid blunders was to write down the planned move on the score sheet, 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. The US Chess Federation also implemented this rule, effective as of January 1, 2007 (a change to rule 15A), although it is not universally enforced.

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