What if a player moves into check without noticing?

Over the board the move is taken back and the position returns to where it was before it. In a standard tournament game the arbiter adds two minutes to the opponent and the game goes on, and a second illegal move by the same player loses. Online the interface simply refuses the move, which is why so many club players have never seen the rule enforced.

Can the king capture a defended piece?

No. If a defender covers the square, the king would be standing on an attacked square after the capture, and that is exactly what the rule forbids. Look at what recaptures before you take with the king. A pawn one square diagonally behind the piece is the defender people miss most often.

Does a pinned enemy piece still stop my king?

Yes. The Laws count a square as attacked even when the attacking piece could not legally move there, because doing so would expose its own king. So an enemy bishop that is pinned against its own king still takes squares away from yours. It is one of the few places where the rules feel unfair, and it is worth knowing before it costs you a game.

Can the king move into check?

Black’s king on e8 is in check from the rook on e1. Four moves get him out: Kd7, Kd8, Kf7, Kf8. The move almost every beginner reaches for, Ke7, is not a legal move at all, because the king would still be sitting on the e-file with the rook staring at him.

The rook on e1 checks along the whole file. Ke7 does not escape it: e7 is attacked just as e8 is. Only the four squares off the file are legal.

Article 3.9 of the Laws puts it in one line: no piece may be moved that leaves or places its own king in check. Not the king, not anything else, not even for one move.

Three ways players break it

Stepping back along the line, as above. A check from a rook or a queen covers the whole file or diagonal, so retreating in a straight line from it is no escape. Step off the line instead.

Capturing a defended piece. This is the expensive one, because the piece looks free.

The knight on d5 sits right next to the white king, and Kxd5 is illegal: the pawn on c6 covers the square.

Moving a piece that was shielding the king. Your bishop on e2 sits between your king on e1 and a black rook on e8, and you spot a lovely square for it on b5. You cannot play it. That bishop is pinned, and a pinned piece cannot move off the line even though nothing in the rules mentions the word pin: the move would leave your own king in check, and that is enough.

The rule counts attackers that cannot move

Here is the clause that surprises people. A square counts as attacked even if the piece attacking it could not legally go there, because moving would expose its own king. So an enemy bishop that is pinned stone dead still takes squares away from your king, and a pinned knight still stops you castling through the squares it covers.

The reverse also holds, which is why a king can never take a king: each one attacks all eight squares around it whether it can move or not.

What happens if you play it anyway

Online, nothing. The board refuses the move and you try something else, which is why so many players reach their first tournament having never seen this enforced.

Over the board, the position goes back to what it was, you make a legal move with the same piece if you can, and in a standard FIDE game the arbiter puts two minutes on your opponent’s clock. A second illegal move loses the game. In blitz it is harsher still: your opponent can claim the win on the first one. So before you play a king move, ask what covers the square. Then look again for the three legal ways out of check, and watch out for pins, which is the tactic that turns this rule into lost material.