with d5

Answering 1. b4 with 1...d5 is the second most frequent reply after 1...e5. Black takes a share of the center with the d-pawn instead of the e-pawn. The trade-off is that, unlike 1...e5, it does not open the f8-bishop's diagonal toward the loose pawn on b4, so there is no immediate tactical pressure on White's queenside. White's usual response by a wide margin is 2. Bb2, putting the bishop on the long diagonal aimed at e5 and the kingside. Black can then develop the light-squared bishop outside the pawn chain with 2...Bf5, the Baltic Defense.

Other second moves are much less common. 2. b5 grabs queenside space at once, and 2. a3 secures the b4-pawn. 2. Nf3 transposes to the Zukertort Opening line known as Santasiere's Folly. 2. e4 leads to the Zilbermints Gambit, catalogued under the Scandinavian Defense, and is the only listed option after which Black scores better. Across the main continuations White wins somewhat more often than Black, and draws are rare. The engine regards the position as roughly balanced.