The Pin
How to spot it: look for enemy pieces standing in front of their king or queen on an open rank, file, or diagonal: then aim a rook, bishop, or queen down that line. Once a piece is pinned, pile more attackers onto it: it can't run.
How to recognise it in a game
Trace every rook, bishop and queen line toward the enemy king and queen. If a lesser piece stands between the long-range attacker and a more valuable target, ask whether moving that piece is illegal or materially costly.
The common calculation mistake
A relatively pinned piece can still move. Treating every pin as absolute causes calculation errors. Count what happens after the pinned piece moves, and remember that a pinned piece may still defend a square even when it cannot legally move to it.
How to defend against it
Break a pin by moving the valuable piece, blocking the line, challenging the pinning piece or adding enough protection to make the exchange harmless. Against a king pin, creating luft or moving the king can restore the pinned piece's freedom.
Now spot it yourself
A real position: find the move. This is exactly what the drills in the app do, banded from beginner to expert.
How this lesson is made: this public explanation comes from the authored Strong Squares curriculum. Automated checks protect its chess data, source fields and learning flow. Independent chess-editor approval remains part of the private-preview release process. Read the teaching method.