The Pin

When two enemy pieces are in a straight line, either on a rank, file or diagonal they can be attacked by pieces that move in that way (Queen, Bishop or Rook). If the enemy piece nearest the attacker cannot move it is said to be pinned.
The Rook cannot move because the King would be in check from the White Bishop. The Rook is said to be PINNED by the Bishop.
White to move. He cannot move the Bishop because the King would be in check from the Black Rook. The white Bishop is PINNED. In this case The Bishop is lost because next move the Black Rook will take it.
White to move. If the White Knight was to move then the Black Queen would capture the White Rook next move. So the Knight is said to be PINNED.
This pinning move can be made extreme;y effective by attacking it a second time, with another piece. Here is a position where a Rook is pinned against the king by a Bishop but there is a nearby pawn! White to move. White could play BxR+ and Black would reply ...KxB. White gains a piece worth 5 Pawns and loses one worth 3 Pawns. This is called 'winning the exchange'. There is a better move for White by attacking the pinned Rook twice he can win this Rook without losing the Bishop at all.
c5! The pinned and helpless Rook is now attacked again by the Pawn.
Black has to move - he moves his King (K-d7 ). The Rook is no longer pinned but it is White's move.
White moves and takes the Rook with his Pawn (c5xd6). White has won the Rook for nothing as theKing cannot recapture the Pawn.

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