Jaenisch Gambit

The Jaenisch Gambit, named after Carl Jaenisch and also called the Halibut Gambit, meets 1. c4 with 1...b5. Black offers the b-pawn so that White's c-pawn, which normally presses on d5, is drawn to the edge of the board. Unlike a typical gambit, the offer gives Black no immediate return for the pawn, and the engine gives White a slight edge. The idea resembles a wing gambit. After 2. cxb5 Black can continue in Benko Gambit style with ...a6. If White takes again on a6, Black recaptures with the bishop or the knight to speed development along the opened queenside files, or leaves the pawn and occupies the center with ...e5.

The acceptance 2. cxb5 is the most common reply, chosen in nearly half of games, and it gives White slightly better results than the alternatives. White can also decline with 2. Nc3, which attacks the pawn, or 2. e3, which opens the diagonal of the light-squared bishop toward b5. The line is a rare surprise weapon. In online play the results are close to even despite the engine's view.