What is Blunder?

A blunder is a catastrophically bad move—a serious oversight that dramatically worsens the player's position, typically by losing material, allowing checkmate, or throwing away a winning or drawn position. In annotation, a blunder is indicated by the symbol ??. Even the strongest grandmasters occasionally commit blunders, particularly under time pressure. A blunder is more severe than an inaccuracy (indicated by ?!) or a mistake (indicated by ?).