Chess Opening Theory/1. e4/1...c5/2. Nf3/2...a6

This seems at first like an unusual move, because Black has now played two pawn moves in the opening and is no closer to developing their pieces. ...a6 is commonly played in the Sicilian defence at some point: for instance, in the main line Sicilian, the Najdorf variation, it comes after 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6. In a sense this is just a shake-up in move order, an "accelerated" Najdorf.

...a6 prevents White from occupying b5. In the Najdorf, Black uses ...a6 to prepare to answer Be3 with ...e5!, and the pawn on a6 prevents the intermezzo Bb5+ (otherwise, 2...Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5 6. Bb5+!, the Venice attack, 6...Nbd7 7. Nf5). If Black omits ...d6 to prevent Bb5 from coming with check (as in 2...Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5), White may still occupy b5 with their knight (6. Ndb5). The O'Kelly variation prepares the ...e5 idea earlier, at the cost of a tempo if White steers away from the typical open Sicilian.

After 3. d4, the so-called "normal" line, 3...cxd4 4. Nxd4 Nf6 5. Nc6 (defending e4; 5. e5? Qa5+!), Black can already play 5...e5!.

White's knight is attacked and must find somewhere to move to.

• b5 is off-limits because of the pawn on a6.
• 6. Nf5? allows 6...d5 and ...d4 to take space and kick White's other knight back to b1 (as 7. exd4?? hangs the knight on f5).
• 6. Nf3 Bb4 pins the knight and threatens to take on e4. If 7. Bd3? d5! threatens ...Bxc3 and ...dxe4, and if 8. exd5?? e4! with a fork. If 7. Bd2 to unpin, 7...d6 defending e5 reinvigorates the threat of ...Bxc3 and ...Nxe4.
• 6. Nb3 Bb4. White also has the option of f3 to defend the e pawn.
• 6. Nde2 is best. After 6...Bb4 7. a3, White is prepared to take back on c3 with the knight and keep e4 defended. Otherwise, 7...Ba5 8. b4 Bb6 9. Ng3.

An improvement is 5. Bd3 to defend e4 instead of the knight, thereby preventing ...Bb5 from pinning e4's defender.

3. Nc3 first guards e4 to prepare opening the centre with d4 & Nxd4. This allows transposition into an standard Najdorf or ...e6 Sicilian.

3. d4 is playable, but by continuing with the plan to open the centre, White lets Black "get away with" an inaccurate move order. The disadvantage of the O'Kelly variation is that if White plays in a way that makes ...a6 less relevant, they have essentially scored a free tempo:

3. c3, the Venice system, is the main move. White is essentially playing an Alapin Sicilian where they got an extra move to play Nf3, because ...a6 is wasted tempo in this line.

3. c4, setting up a Maróczy bind, is most critical. White uses the free tempo to play c4 before Nc3. The main continuation is 3...Nc6 4. d4 cxd4 5. Nxd4 e5.

For explanation of theory tables, see theory table and for notation, see algebraic notation.

1.e4 c5 2.Nf3 a6

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Wikipedia has related information at Sicilian Defence.

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