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

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 bishop with 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 they can move his knight 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 typically prefer to unpin the knight immediately with 4...Be7, or to deny White the opportunity of playing 5. e5 with 4...dxe4.

The only major variation where Black ignores the potential dangers of the pin is the McCutcheon variation: 4...Bb4.

1. e4 e6 2. d4 d5 3. Nc3 Nf6 4. Bg5

Wikipedia has related information at French Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.