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Definition
Check is when the king is threatened by an opposing piece.
When in check the king can either:
- move to a safe square, or
- capture the piece that is threatening him.
It may be possible to place a piece between him and the threatening piece. That is called interposing.
If he cannot do any of these things, then it is checkmate and the game is over.
In the example on this page after the king is placed in check he can move to safety to square f8.