Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nf3/3...Nf6/4. e3/4...Bg4

Black develops the bishop to g4, pinning the f3 knight to the queen. White will generally want to get rid of this pin quickly, and has many ways to do so.

5. h3 immediately asks Black the question of whether they want to exchange their bishop for the knight. Both 5...Bh5 and 5...Bxf3 are viable options. After 5...Bh5 White will usually strike back with g4 soon.

5. Nc3 doesn't address the pin yet, but White will likely play h3 soon, often transposing into lines where White begins with h3 instead.

5. Qb3 breaks the pin by moving away the queen, threatening to take the b7 pawn. After either 5...Qb6 or 5...Qc7 White has the looming threat of playing Ne5 with tempo on Black's bishop.

5. cxd5 exchanges the central pawns, resembling the exchange Slav. Black usually responds by trading off the f3 knight, which has the potential of becoming an important attacking piece when transferred to e5.

1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. e3 Bg4