At attack by a queen, rook or bishop on a piece which cannot move without exposing a more important piece or square. The pinned piece is said to be masking or screening the more important piece or square. An absolute pin is where the screened piece is a king, therefore it is illegal for the pinned piece to move as it would expose the king to check. A relative pin is where moving the pinned piece would result in a loss of material or other unfavourable effects.

An attack (by a Rook, Bishop or Queen) on a piece that cannot or should not move, because a piece behind the attacked piece is worth even more. If the piece behind is a King, this is an “absolute” pin and the pinned piece is not allowed to move, or it would put the King into check.

When a piece is attacked but cannot legally move, because doing so would expose the player's own king to the attack; or when a piece is attacked and can legally move out of the line of attack, but such a move would expose a more valuable piece (or an unprotected piece) to capture. See absolute pin and relative pin, respectively.