Blunder check
A blunder check is a quick test, made by a person before moving or by software afterward, that looks for a move which hangs material, allows a tactic or loses the game outright.
Also called: blunder check tool
A blunder check works in two settings. Over the board, it is a short pause before you release a piece: you ask what the move leaves undefended, what checks and captures your opponent now has, and whether the piece you are moving was guarding something. After the game, a software blunder check scans your moves and flags the ones where the evaluation fell sharply. The over-the-board version is the more valuable skill, because most club games are decided by one oversight rather than by deep strategy. A common mistake is checking only your own plan and never asking what the opponent's last move threatens. Another is skipping the check when the move feels obvious, which is exactly when a recapture, a pinned piece or a back-rank problem slips through. A practical habit is a fixed routine: look at every check, capture and threat for the opponent after your intended move, then play. Keep it brief enough to repeat every move. Software tools differ in how big a drop counts as a blunder, so treat their labels as a guide rather than a verdict.
Compare it with Blunder check habit. A blunder check habit is a short, fixed routine run before every move, asking what the opponent threatens and whether the intended move leaves anything undefended, to catch one-move oversights.
Also worth reading is Fair play. Fair play is the rule that every game must be decided by the players' own thinking, which means no engine help, no advice from others during play and no tricks that distort a result.
A neighboring idea is Premove. A premove is a move entered on an online board before the opponent has replied, which is played automatically as soon as it becomes your turn if it is still legal.
An engine can tell you what "Blunder check" means for a position, but not which mistakes you keep making; a first lesson with George Li reviews your games and finds those.
Related terms
A blunder is a very serious error, such as leaving a piece undefended or missing a decisive tactic, that swings the evaluation so far that a good position turns bad or a bad one becomes lost.
A hanging piece is a piece that is attacked by the opponent and has no defender, so it can be captured for free unless its owner moves it, defends it or makes a bigger threat.
A loose piece is a piece that no friendly piece or pawn protects, which makes it a target even when nothing attacks it yet and gives the opponent a chance for a double attack.
A blunder check habit is a short, fixed routine run before every move, asking what the opponent threatens and whether the intended move leaves anything undefended, to catch one-move oversights.
Candidate moves are the small set of plausible moves you shortlist in a position before analyzing any of them in depth, so that your thinking is organized and no strong option is overlooked.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
Want to spot this in your own games?
A first lesson starts with your games: George reviews them and builds a study plan around what he finds.