Chess Opening Theory/1. e4/1...e5/2. Nc3/2...d6

2...d6 is a minor sideline. Black, unprovoked, defends his e5 pawn and in doing so restricts his king's bishop from entering the game and does not challenge White's control of the centre.

Black adopts the same pawn structure as in the Philidor defence (1. e4 e5 2. Nf3 d6), and as in the Philidor, the critical move is to blow open the centre with 3. d4. White is still ahead even though Black gains tempo on their queen in this line: 3...exd4 4. Qxd4 Nc6 5. Qe3 Nf6 6. Bd2 Be7 7. O-O-O O-O⩲.

3. f4 is the Omaha gambit, where White attacks the centre from the flank. It leads to king's gambit-like positions.