Chess Opening Theory/1. e4/1...Nf6/2. e5/2...Nd5/3. c4/3...Nb6/4. d4

Two pawns attack B02 Alekhine's defence → Two pawns attack 4. d4 Black would now like to carry on with their development, but they find White's pawn chain to be stifling. The main move is 4...d6, because Black is left with little other choice. They cannot play 4...Nc6?, because 5. d5! Nxe5 6. c5 Nxd5 (6...Nbc4 7. Qd4!) 7. Qxd5 ++, and 4...Na6?! is grim, little better than not developing the knight at all. Instead Black looks at pawn moves, which may allow them to develop their bishops. 4...e6?! may seem natural, but developing their king's bishop leaves Black's king's side too vulnerable: 5. Nc3 Bb4? 6. Qg4! and that bishop will have to go right back to f8 (6...O-O? 7. Bh6! and Black will be very unhappy to have to play g6). 4...g6?! and 5...Bg7 keeps the bishop in defence of the king's side, but it's prospects are poor while there is still a pawn chain on d4 and e5. Therefore, Black plays 4...d6. This increases their queen's bishop's prospects, with the added advantage of attacking the problematic (and, Black argues, overextended) pawn chain. If Black can carry out the trade ...dxe5, their queen will also be able to control d5 and so Black will be safe to develop ...Nc6 (as, in this line, d5 Nxe5 c5 Nxd5 and the knight is defended). Theory table 1.e4 Nf6 2.e5 Nd5 3.c4 Nb6 4.d4 4 5 6 ...d6 exd6exd6 Nc3Be7 = ...Nc6?? d5!Nxe5 c5!Nbc4 f4 +- Black loses a knight. References Alekhine Defence