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An attacked piece is trapped, when the attack cannot be defended without losing material.

Explanation

In "Fundamentals" we saw many ways to defend an attacked piece, including running away to a safe square, blocking the attack, and protecting the piece. In the defence course, several other methods can be learned, including defending by check or capture, and counter attacks.

Sometimes none of these methods are possible, or only possible under material losses, and a piece attack immediately wins material. The piece is trapped.

Examples

White can trap Black's bishop d6 with c5.
The bishop has no safe square to escape to.

Advanced: Black's last move was 9...Bb4-d6, after White had just attacked it with 9.a3.
Black could have saved the bishop instead with the intermediate move 9...dxc4, attacking White's bishop on d3, and returning to d6 after White recaptured on c4.
Note that the retreat 9...Ba5 would not have saved the bishop.
10.b4 Bb6 11.c5 would trap the bishop on b6, however after 10.b4 Black has the same intermediate move 10...dxc4.
But 10.c5! would trap the bishop anyway, Black has no defence against 11.b4.

White's queen has advanced deep into enemy territory, but it has run out of squares.
Black can trap the queen with Ne6!

Sometimes pieces can get trapped even in the endgame:

Black can trap the bishop with Kc7!

Advanced: On the previous move, White had a surprising way to defend against the trap.
See Puzzle 119298.

Visualization

If a piece has no safe squares to go to, always be alert for ways to attack and trap it.