Moves You Cannot Play

Some moves look normal and are simply not allowed, and they all protect the same person. Rule one: the king may never move onto a square an enemy piece attacks, because that would be walking into check. Here the black rook on a2 sweeps the entire second rank, and the white king's three forward squares are all off limits:

The rook on a2 guards the whole rank. The king on e1 may only go to d1 or f1; stepping to rank 2 would be stepping into check.

The same rule produces a fact you can see in every endgame: two kings can never stand on neighboring squares. Each one attacks the ring around himself, so approaching would be moving into the other's attack. These kings are as close as the rules allow:

Kings on e4 and e6. The squares d5, e5 and f5 belong to both kings' attack rings, so neither may ever step there.

Rule two hides better. A piece standing between an enemy attacker and its own king is pinned: moving it would expose the king to check, so it may not move. This knight on e4 shields its king from the rook on e8, and every knight move leaves the e-file, so the knight cannot move at all:

The rook on e8 pins the knight on e4 to the king on e1. The knight is frozen: any jump would leave the king in check.

Both rules come from one principle: no move may leave your own king in check. If a move would, the move does not exist, no matter how tempting the capture on the other end. The board is full of moves; these were never among them.