Chess Opening Theory/1. d4/1...Nf6/2. c4/2...c5/3. d5/3...b5/4. cxb5/4...a6

Black offers the a-pawn to White.

5. bxa6 fully accepts the Benko gambit. This consolidates White's material lead. They have taken two pawns and Black can only recover one (either right away, 5...Bxa6, or later after developing the king's side). This is a positional sacrifice; in return, Black gets open a- and b-files to work with and one solid pawn island.

5. Nc3, the Zaitsev system, is a sharp alternative. This allows 5...axb5. Though White can take it again with 6. Nxb5 and be up a pawn, the usual idea is 6. e4, supported by the knight, to take space and support the d5 pawn. 6...b4 7. Nb5 is the Nescafé frappé attack (yes, really), which sets a cunning trap. Did White just blunder their e-pawn? 17...Nxe4? 8. Qe2! Nf6?? 9. Nd6#.

5. f3, the Dlugy variation, prepares to play e4 supported by the f-pawn instead. 5...axb5 6. e4 Qa5+ 7. Bd2 b4 8. Na3 d6 9. Nc4 Qd8 =.

5. e3 is the modern variation. This opens White's bishop to defend the pawn, 5...axb5 6. Bxb5 and a testing option involves play around White's loose d-pawn with ...Bb7, ...Qa5 etc, e.g. 6...Qa5+ 7. Nc3 Bb7 8. Bd2 Qb6 (these queen moves induce Bd2 to prevent White's queen from seeing d5) 9. Nf3 Nxd5 10. a4 e6.^[1] Otherwise, Black often leaves the tension and proceeds with 5...g6, ...Bg7, ...O-O, ...Bb7.

5. a4? to hold onto the pawn fails as after 5...axb5 the a4-pawn is pinned to White's rook.

5. b6, the pawn return variation, declines the gambit and allows Black to regain the pawn immediately with 5...Qxb6. This avoids complications and so is a good line for players who don't wish to get involved in theoretical lines as White.

1. d4 Nf6 2. c4 c5 3. d5 b5 4. cxb5 a6

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

Wikipedia has related information at Benko Gambit.