When a player moves a piece to a position where it directly threatens the opponent's king, that king is said to be in check. The player putting the opponent's king in check must announce, "Check." The player with a king in check must respond by escaping check. There are three ways to escape check:
• capture the checking piece
• block the checking piece
• move the king out of check
In this example, a Black knight puts the white king in check. Note that the knight is also threatening the queen. White has no piece that can capture the checking knight, and knights cannot be blocked. White's only option is to move the king out of check, and the only available escape is to b2.