Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nf6/4. O-O

Castling is the most common continuation in the Berlin defence. This leaves White's e4 pawn undefended, but gives White's rook access to the centre.

If Black accepts the pawn, 4...Nxe4, this is actually a trade because White can take back e5 easily. One line is 5. Re1, bringing another attacker to the e-file and recovering the pawn after the knight is forced to move. One of the reasons this works is because Black's king is still in the centre: 4...Nxe4 5. Re1 Nd6 (threatens to take White's bishop) 6. Nxe5 Nxe5 (Black is not in time to capture the bishop: 6...Nxb5, because the discovered check Nxc6 wins White the queen) 7. Rxe5+ Be7 and White can evacuate the bishop.

5. Re1 is very drawish, and 5. d4 is actually the main line (although as this is the Berlin, 5. d4 is pretty drawish too, and usually leads to the Berlin Wall endgame). 5...exd4? opens the e-file and lets 6. Re1 pin-and-win Black's knight, so the main reply is 5...Nd6.

4...Bc5, the Beverwijk variation, is the main alternative to 4...Nxe4. Black pressures f2 and White's castled king. This is sharp because 5. Re1?! to defend e4 undefends f2 so after 5...Ng4, White finds themselves having to move it back 6. Rf1. (A trap is 6. d4?? Nxd4! 7. Nxd4 Qh4!-+).

White has the choice to capture e5 (5. Nxe5), due to a tactic that arises from the c5 and e5 squares being forkable. More commonly White plays for c3 and d4 instead.

4...Be7 before taking the pawn shelters Black's king and gets ready to castle. This prevents any mischief around a discovered check. Now the threat of 5...Nxe4 is more severe, as while White can still recover the pawn, they would have to give up the bishop pair to do so (e.g. 5. c3?! Nxe4 6. Re1 Nd6 7. Bxc6 dxc6 8. Nxe5 Bf5). 4...Be7 functionally declines the gambit, as it gives White time to play 5. Re1 and defend the e4 pawn. It is typically a quieter continuation.

4...Ng4?!, the Fishing pole variation, invites White to blunder by accepting a sacrificial knight but opening the h-file and exposing themselves to potential checkmate.

For explanation of theory tables, see theory table and for notation, see algebraic notation..

1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6 4.O-O

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