Discussion: Pinning Part I

There are 3 elements to a pin: the pinning piece (green squares), the pinned piece (yellow squares), and the screened squares (blue arrows). This is an important point in identifying pins; there may or may not be an opposing piece standing on one of the screened squares, there may be a different threat such as delivering mate on one of them.

White Bishop Pins Black Knight, Black Rook Pins White Rook

Technically x-rays until the screened squares attain higher target value

There may be no threat associated with any of the screened squares. If the value of a target on any of the screened squares is less than the value of the pinned piece, those squares are technically speaking x-rayed. The value of the pin depends on the value of the screened squares, and the mobility of the pinned and any screened pieces.

Reinfeld position #2
White to move

Here the Black Knight is pinned by the Bishop to the value screened square d8, and by the Queen to the value square g7. This is an absolute pin, as the Knight may not legally move. The pin is exploited by a decoy sacrifice that allows white to infiltrate the Black squares surrounding the King. 1. Re8 Qxe8 2. Qxf6+ Kg8 3. Bh6 followed by mate at g7 or f8.

Reinfeld position #431
White to move

We commonly associate pinning as being between different types of pieces; Bishops pin Knights for example. This diagram illustrates pinning between pieces operating along the same line and therefore mutually attacking. 1. Bf6 gxf6 2. exf6 Rg8 3. Rd8 the function of this pin is to neutralise the g8 Rook's protection of the mating square g7 so after 3. ... Rcxd8 4. Rxd8

The pin by mutually attacking like pieces
Reduces the pinned piece's mobility (g7)

5. Qg7 will be mate whether the Black Rook moves or not.

Knight pinned to screened mate threat
White to move

There is no piece on the screened squares, the value-target is a mate-threat. After 1. e6 the Knight is lost, for if it moves, 2. Rg7+ Kh8 3. Rh7+ Kg8 4. Rcg7#.