Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nc3/3...Bb4

3...Bb4 develops a piece and threatens to take or pin (should White move their d-pawn) the knight, thereby pressuring White's control of the centre.

4. Nxe5, taking the e5 pawn, is the main move. With their e4 pawn defended, White should feel confident that Black cannot take back immediately, and if Black thinks to remove the defender of e4 to take back the pawn, 4...Bxc6 5. dxc6 Nxe4?, this loses to 6. Qg4!, forking the knight on e4 and the g7 pawn. (6. Qe2, as in 3. Nxe5 Nxe4 4. Qe2, is also viable.)

However, White should allow Black to recover the material eventually. First Black should castle, 4...O-O, which defends g7 so threatens to take the pawn. It is inaccurate for White to attempt to hold onto the material: 5. d3?! d5! threatens ...dxe4 dxe4 Nxe4 (White's knight is now pinned):

• 6. Bd2 (to unpin the knight) Re8 7. Nf3 Bxc3 8. Bxc3 dxe4 9. dxe4 Qxd1+ 10. Rxd1 Nxe4⩱, Black has the material back and strong initiative.
• 6. exd5?? is a common blunder. 6...Nxd5 threatens to take twice on c3 and win the exchange. 7. Bd2 Bxc3 8. bxc3 Re8∓ and White succumbs to pressure on the open e-file and their uncastled king.

The main line is 5. Be2 (preparing to castle) Re8 6. Nd3 (a small optimisation over Nf3: the d-pawn wasn't going anywhere and White gets to keep the move f3 to kick the knight later) Bxc3 7. dxc3 Nxe4 (giving back the pawn) 8. O-O⩲.

4. Bc4 is also played.

1. e4 e5 2. Nf3 Nf6 3.Nc3 Bb4

Wikipedia has related information at Petrov's Defence.

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