Interposition

Check means that the king is attacked and must be made safe on the next move. When a rook, bishop, or queen checks from a distance, its attack travels along a row, column, or diagonal without jumping over pieces. Interposition means moving another friendly piece onto an empty square between the attacker and the king, breaking that line.

Three answers to a single check

1. Move the king to a square that is not attacked.
2. Capture the checking piece, provided the king is safe afterward.
3. Interpose a piece when the check travels along a line and at least one square lies between the attacker and the king.

A knight, pawn, or king check cannot be interposed against because those attacks do not travel along a blockable line. A rook, bishop, or queen checking from an adjacent square also leaves no square on which to place a blocker.

Interposition cannot answer a double check either. Blocking one attack would leave the other active, so the king must move. Against one distant line check, however, a blocker may buy time even if the attacking side can capture it afterward.