Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O/5...Be7/6. Re1/6...b5/7. Bb3/7...O-O/8. c3

Closed Main Line, (to) Marshall Gambit
Black can now pull the trigger on the Marshall Gambit, sacrificing a pawn to quickly activate all her pieces while stripping the White kingside of defenders, with
 * 8...d5

Alternatively, transposes back to the Closed Main Line (7...d6 8.c3 O-O). This move is actually more common than the Marshall Gambit, although it's the Marshall Gambit which makes 7...O-O into a significant alternative to 7...d6.
 * 8...d6