Once I encountered an opponent who used a very early a6 in his Sicilian Opening.
[FEN ""] 1. e4 c5 2. Nf3 a6
What could have been his objective with this move and how should I have have countered it?
This is called the O'Kelly variation of the Sicilian.
If white plays normally (i.e.
3.d4 cxd4 4.Nxd4 Nf6 5.Nc3), then black can eventually kick the d4 knight with
...e5 when the knight doesn't have any great squares. In this case,
2... a6 keeps the knight from moving to the natural b5 square (as in the Sveshnikov Sicilian). This will allow black to quickly play
...d5 with equality.
White's best plan is to play either
c4 trying to prove that black's move
2... a6 is not a good developing move. When white plays
c3, the natural plan is to follow up with
d4, and then if black plays
...cxd4, white will respond with
cxd4, gaining a classical pawn center. When white plays
c4 instead, white's plan is to set up a Maroczy bind.
An unambitious plan for white would be to play into a closed Sicilian - in the closed Sicilian, black frequently plays a6-b5 in order to get counterplay on the queenside. Furthermore, in the closed Sicilian, tempi matter a lot less because the position will not change radically from one move to the next.
As I understand it, the purpose of
2... a6 is to keep a white piece (usually knight or bishop) off b5.
It seems premature, because white might not want to go there anyway (for some time). Meaning that it could easily become a wasted move.
As White, I would concentrate on king side development, knowing that b5 was "off limits," but not caring, because I have "better" things to do.
One disadvantage of a6 is that Black may want to play a5 later to try to "chase" a White knight on b3. But if he does so, he would have wasted a move with a6. As White, I might try to "transpose" into that variation so that Black has wasted a move.
The point of ...a6 is to control the square b5 where white can otherwise place a knight or bishop at some point. Another point is to prepare b7-b5 followed by Bc8-b7 and perhaps b5-b4 to chase a white Nc3. The move ...a6 appears in at least two variations, 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6; 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 a6. If you have ever been interested in the gambit 1.e4 c5 2.b4, then you can consider 1.e4 c5 2.Nf3 a6 3.b4.
[FEN ""] 1.e4 c5 2.Nf3 a6 3.b4 cxb4 4.a3 bxa3 5.Bxa3
a6 serves three purpose .