A Pin is a tactic used in Chess which restricts the mobility of an enemy piece. It occurs when an when an attack is created against a piece, and that piece isn't able to move without exposing a more valuable piece behind on it (on the other side). That could be captured. There are two main types of pins: The first one is where the attacking piece is attacking a lower valued piece, and directly behind is the king. In this situation - it is illegal for the piece under attack to move since it would expose the king to check. The second type of Pin is where a piece is under attack, and directly behind is a higher valued piece other than the king. The piece under attack can technically move - but would be undesirable.