Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nc3/3...Nf6/4. Bg5

=Classical variation= Although the pin looks like White is threatening to win the knight by playing 5.e5, Black can defend against this threat by counterattacking the Bg5 (5...h6!). In order to chase the bishop away after White's 6.Bh4 Black may well be forced to follow up with 6...g5, and then he can move his Nf6 to a safe square. Although this way of defending against the pin may be sufficient to avoid the loss of a piece, the maneuver 5....h6 followed by 6...g5 has the major drawback that it weakens Black's kingside. Hence after 4.Bg5 Black players usually prefer to unpin the knight immediately (4...Be7), or to deny the opportunity of playing 5.e5 (4...dxe4) to White. The only major variation where Black ignores the potential dangers of the pin is the McCutcheon-Variation: 4...Bb4.

Theory table
.