Key ideas
- The Berlin endgame comes after 4.O-O Nxe4 5.d4 Nd6 6.Bxc6 dxc6 7.dxe5 Nf5 8.Qxd8+ Kxd8. Black has lost the right to castle and has doubled c-pawns, but holds the bishop pair and a position with very few targets.
- In that endgame White has a healthy kingside majority and the e5 pawn cramps Black. White plays Nc3, h3, Rd1 and looks for a kingside pawn advance; Black tucks the king on c8 or e8, coordinates the bishops and uses ...h5 or ...h6 to restrain White's pawns.
- Players who want to keep queens on choose 4.d3, the Anti-Berlin. It defends e4 and leads to slow, Italian-like manoeuvring with c3, Nbd2-f1-g3 and sometimes Bxc6 at a good moment.
- 4.O-O Nxe4 5.Re1 Nd6 6.Nxe5 is another way to avoid the endgame, and 4.O-O Nxe4 5.d4 Be7 is Black's older, queens-on alternative to 5...Nd6.
Plans for White
In the endgame, play for space and slow pressure: Nc3, h3, Rd1 and a timely kingside pawn push, keeping the e5 pawn as a wedge. If that does not suit you, 4.d3 keeps queens on with little risk.
Plans for Black
Know the endgame plans: bring the king to safety on c8 or e8, coordinate the two bishops, and restrain White's kingside majority. In the 4.d3 lines, ...Bc5, ...d6 and ...O-O give a sound position.
Watch out
After 4.d3 Ne7 (the Mortimer Variation), the pawn grab 5.Nxe5? runs into 5...c6. The bishop must move, and if it retreats, 6...Qa5+ forks the king and the e5 knight. White ends up with two pawns for a piece at best.
For the wider picture, see the Ruy Lopez guide.