Western, Rhamphorhynchus

This line arises from the Sicilian move order. Instead of the usual Open Sicilian reply 3...cxd4, Black plays 3...Bg7, letting White take on c5. After 4. dxc5 Qa5+ White is a pawn up for the moment, with doubled c-pawns, but the check gains Black time and the queen is aimed at the c5-pawn. Black usually expects to regain it, keeping the g7-bishop on the long diagonal and gaining a half-open c-file. The engine gives White a slight edge. In Lichess games the results are close to even.

White has three main ways to block the check, and each is played about equally often. 5. Nc3 leads to the Sicilian Rhamphorhynchus. There the knight is pinned by the queen and exposed to the bishop, and the Pteranodon, 5...Bxc3+ 6. bxc3, becomes possible. 5. Bd2 blocks with a piece and avoids the pin. 5. c3 uses a pawn, which also blunts the g7-bishop's diagonal. Engines also consider 5. Nfd2, which keeps the c-pawns intact and the long diagonal free of targets on c3.