Here's a study in the defensive side of our current theme. Black sees that White’s pawn on c5 is attacked twice and protected just once. He's tempted to play Rxc5, picking up the pawn for nothing since White doesn’t dare recapture. But the thing to see is that the contested square—c5—is on an open diagonal with Black's own king. As soon as he plays Rxc5 his rook is poised to be pinned. White then has Qb4, and now it's Black’s rook that is attacked twice and protected only once—and paralyzed to boot. It is lost next move. The point of the position is to think carefully when you are looking at a possible series of exchanges and tallying up how many times a square is attacked and defended. Sometimes a piece not currently putting pressure on the square can swoop in afterwards and impose a pin that changes everything. The time to worry most about this, of course, is when the contested square is on a line with either king.