Chess Opening Theory/1. e4/1...d5/2. exd5/2...Qxd5/3. Nc3/3...Qa5

Black moves the queen to a5. This is a good square for the queen: it can't be easily attacked again, it supports Black playing ...e5, and will pin the knight if White advances their d pawn. Later it can move to b6 to target the b2 pawn.

The main line continues 4. d4. While this pins White's own knight, it frees the d2 square for the bishop, so that White might move their knight with tempo on the black queen. White makes typical developing moves like Nf3 and Bc4 in one of several move orders that are largely equivalent. Black typically develops with ...Nf6 and either ...Bf5 or ...Bg4 (routing to g6 if challenged with h3 Bh5 g4 Bg6) and includes the move ...c6 to provide an escape route for their queen.

4. b4!? is the Leonhardt gambit. White gives Black the b-pawn in exchange for some tempo to developed. White gets good compensation for the pawn, with engines evaluating the sacrifice as around even, and there are some sharp lines that offer White good attacking chances. Historically, Lasker, Capablanca, and Keres have attempted this line with decent success.^[1][2][3]

Preparing 5. b4 with 4. Rb1?! fails to 4...c5.

1. e4 d5 2. exd5 Qxd5 3. Nc3 Qa5