Flight square

In chess, a flight square or escape square is a safe place or a square to which a King or other piece can move if it is threatened. A bishop sometimes begins to get hemmed in after Morphy's defense is used, and thus pawn to c3 may be used to create an extra escape square.

One way to get a king out of check is to move to a flight square on the next move. (The other ways to get out of check are to capture the checking piece or to interpose a piece to block the check.) If the checked king has no flight square and there is no other way to get out of check, it is checkmate.

One way to win material from an opponent (that is to say, end up with more pieces or more valuable pieces left on the board) is to dominate a piece by removing all of its flight squares (through attacking or occupying them), then threatening to capture it.

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Wikipedia