A Pin occurs when a piece is attacked (but not necessarily threatened with capture) by an enemy unit, and is prevented (or discouraged) from moving off the line-of-attack because doing so would reveal an attack on a more valuable piece.

An Absolute Pin arises when the target behind the pinned piece is the King; in this case, the pinned piece cannot move because it would reveal a check - an illegal move

Often the pin itself may not cause direct damage, but many tactics can spring from it

Pin Breaking Opportunities

• always look out for pin-breaking opportunites. These are especially common when a Bishop pins a Knight to the Queen and is more likely to work if the pinning piece is unprotected
• attacking the pinning piece directly can sometmes allow the defender to wriggle free - most commonly moving pawns to chase away an advanced Bishop

Common Pin threats

• always be wary of open lines and diagonals towards the King, such pins can easily become deadly
• when a pin threatens to win a piece, always count how many times it can be attacked and defended; there may be a win of material if the pinned piece can be attacked more times than it can be defended

Pinned Pieces do not defend

• always remember that a pinned piece does not defend, it is frozen to the spot unless there is an opportunity for pin-breaking. This means that you cannot consider moving to squares that at first glance seemed protected