Pins: why a knight has ordinary moves but no legal ones
A knight can jump, yet sometimes cannot leave for any of its usual squares. Obstacles are not the problem: its king stands behind it. A jump exposing the king to attack is forbidden. Pins reveal an opponent's restrictions and explain why your own piece may suddenly be unable to move.
In this article
Find the attacker, blocker and target
Along the e-file, White's rook is on e1, Black's knight on e7 and king on e8, with the intervening squares clear. While the knight stays on e7, the rook cannot reach the king. Any knight jump removes the only blocker and exposes check. The knight's geometric possibilities are therefore not legal moves here. King safety takes priority.
Remove the cause
Black plays e8–f7, taking the king off the dangerous file. A knight move no longer exposes that king to check. White's illustrative h2–h3 keeps the rook in place; afterwards e7–c8 is legal. The pawn reply makes the comparison clear, rather than promising the best defence: White could keep applying pressure in a real game. This demonstration explains legality, not a forced win.
Distinguish forbidden from costly
A pin to the king is absolute. If a queen stands behind the pinned piece, moving may be legal but expensive: a relative pin. Even an absolute pin need not mean total immobility. A rook or bishop can sometimes move along the line while shielding the king, or capture the attacker. Do not generalise this knight's restriction to every piece.
Exercise: explain each legal move
Before the first step, select e7 and explain the absence of knight moves. Check again after the king moves. In your own position, replace the knight with a black rook and inspect its moves along the e-file: some retain the shield. Before a knight jump, ask which line it opens. To break a pin, consider moving the king, blocking the line or capturing the attacker, and check each option across the whole board.
Your move now.
Practise the idea on the board. You can make mistakes, try again and start over.
Open exerciseRules and sources
Use these links to check the rules and explore the topic further.