Cunningham Defense, Bertin Gambit

Instead of stepping aside with 5. Kf1, White meets the Cunningham check 4...Bh4+ by interposing 5. g3. This offers a second pawn in order to keep the right to castle. Black almost always captures with 5...fxg3, and the gambit proper continues 6. O-O gxh2+ 7. Kh1. The alternative name, Three Pawns' Gambit, refers to White having parted with the f-, g- and h-pawns by this point. In return White's king is tucked away and White is ahead in development. The rook on f1 also gains the open f-file, and together with the bishop on c4 it bears down on f7. The line dates from the 19th century and seldom appears in serious modern play.

The engine gives Black a slight edge. This fits the verdict that White's compensation is considerable but falls short against accurate defense. Online results point the other way, with White scoring well from this position. Declining the extra pawn with 5...Be7 or 5...Bg5 is rarely chosen.