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2.g3. This fianchetto of the KB signals White's attempt to control the central squares. It is the main line for tournament players. White continues with moves like Bg2, Nf3, 0-0, c4, Nc3.
2.Bg5. This is a hard move to understand. It makes 2...Nf6 doubtful, since 3Bxf6 doubles pawns and makes Black's game more difficult. Black usually plays 2...g6 to fianchetto his bishop before he plays ...Nf6. 2...g6 marks the Leningrad variation of the Dutch defence. The bishop cannot be chased successfully: 2...h6 3.Bh4 g5 4.Bg3 f4? 5.e3! threatens mate, and so wins the pawn on f4.
2.e4. This is Staunton's Gambit. After 2...fxe4 3.Nc3 Nf6 4.Bg5 d5 5f3 White may or may not have enough for the f pawn.