Blunder, mistake, inaccuracy: what the review means

A blunder is a move that throws away something serious. In a game review it is not a judgement about you, it is a threshold being crossed, and knowing which one makes the report far more useful.

Written ?? after the move
In a review an evaluation drop past a threshold
Thresholds vary by site and version

A blunder is a bad move of the serious kind: one that hangs a piece, walks into mate, or turns a winning position into a lost one. In notation it is written with a double question mark after the move, as in 6.Nxd5??, while a single question mark is a mistake and ?! is an inaccuracy.

Those three words have a specific meaning in a computer game review, and it is not the meaning people assume.

What the three labels actually measure

An engine scores a position in centipawns, where 100 means roughly one pawn. A review takes the score before your move and the score after it, and looks at how far the position dropped. A small drop is an inaccuracy. A larger one is a mistake. The largest is a blunder.

The cutoffs between them are chosen by whoever wrote the review tool. They are not a fact about chess, they differ between sites, they change between versions, and some tools convert the evaluation into a winning percentage first, which is why the same move gets called a mistake in one place and a blunder in another. Do not read the label as a verdict. Read it as a pointer to the move worth looking at.

Two consequences follow. A blunder in an already lost position often changes nothing that matters, and a move flagged as a blunder in a sharp position is sometimes a perfectly reasonable practical try that only an engine can refute.

A real one, from a real opening trap

From the first move. A Queen's Gambit Declined in which Black appears to leave a pinned knight and a pawn hanging.

1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 Nbd7 5.cxd5 exd5 6.Nxd5 Nxd5 7.Bxd8 Bb4+ 8.Qd2 Bxd2+ 9.Kxd2 Kxd8

Black has won a knight for a pawn. White's blunder was 6.Nxd5, played because the f6 knight looked pinned. It was pinned against the queen, not the king, and a pin like that is only a suggestion.

The three things that actually cause blunders

First, you moved a defender. This is the biggest single cause by a wide margin. A piece that was guarding something goes off to do something else, and the thing it was guarding is now en prise. The move that loses the material is not the move that puts the piece in danger.

Second, you answered the threat you already knew about. Your opponent moves, you deal with what they threatened last time, and you never ask what the new move changed. Almost every unexplainable blunder is a reply to a move nobody actually looked at.

Third, you stopped looking once you found something good. A move that wins a pawn feels finished, so it never gets the final check against your opponent's reply. Low clock makes all three worse, which is an argument for a short fixed routine rather than a better one.

Before every move: what does my opponent's last move now attack, and what did it stop defending?
After choosing a move and before playing it: is the piece I am moving defending anything, and is the square I am moving to defended?
Look at every check your opponent has in reply. There are rarely more than three and one of them is usually the refutation.
Practise the pattern, not the resolve. Nobody has ever stopped blundering by deciding to concentrate harder.

Questions people ask

What is the difference between a mistake and a blunder in chess?

Size. Both are moves that make your position worse; a blunder makes it much worse. In a game review the difference is a numerical threshold in how far the engine's evaluation dropped, chosen by the tool rather than fixed by chess, so the exact boundary varies between sites. In ordinary conversation, a blunder is the one that loses a piece or the game.

What does ?? mean in chess?

Two question marks after a move mean a blunder. One question mark means a mistake, ?! means a dubious or inaccurate move, ! means a strong move and !! a brilliant one. The symbols are old annotator's shorthand and predate engines by more than a century, which is why they describe how bad a move was rather than by how much.

How do I stop blundering pieces?

Build one habit, not five. Before each move, look at what your opponent's last move changed and then check whether your intended move leaves anything undefended. That single check catches the large majority of them. Drilling undefended pieces as a pattern helps too, because recognising a loose piece instantly is faster than reasoning about it.

Blunder sits under Words you hear at the board in the reference. Nearby: