Black plays 5...a6, entering the Najdorf Variation, which is the most popular variation of the entire Sicilian Defence. It is variation is named after the Polish-Argentinian Grandmaster Miguel Najdorf. With this move, Black controls the b5 square, preventing any of white's pieces from occupying that square, and thus prepares 6...e5 to kick the knight from d4, while preventing 7.Ndb5 or 7.Bb5+. It also supports further queenside expansion with ...b5 and b4 in the future, which could become dangerous for White if he castles queenside, as he does in a lot of variations.

Flexibility is a key to this opening, as White doesn't have any immediate threats, Black maintains flexibility with the b8 knight, developing it to d7 in most lines, but also c6 in a few lines, as well as the c8 bishop, which can either go to b7, though usually to e6. Black also maintains the flexibility of where to move the f8 bishop, either to e7 if the e-pawn is advanced to e5 or e6, or g7 if the bishop is fianchettoed, which happens in many variations.

However, Black's move 5...a6 doesn't make any immediate threats, therefore White has a large variety of ways to continue:

The most popular move is 6. Bg5, developing the dark squared bishop actively and threatening to double Black's pawns with an eventual Bxf6. Black usually responds with 6...e6, entering a Scheveningen-type pawn structure as playing 6...e5 would leave the d5 square too weak, with White having far more control of the square compared to Black.

White can also develop the bishop to e3 with 6. Be3, known as the English Attack. With this move, White aims to follow up with Qd2 and O-O-O, and eventually play f3, g4, and h4 aiming for a quick kingside attack. This usually results in opposite-side castling, with a lot of dynamic play and chances for both sides.

White can also play 6. Be2, which prepares castling kingside on the next turn. With this move, White abandons playing for the kingside, and instead rather aims for a more calmer positional game, usually focusing on the weak d5 square if Black plays 6...e5. If Black plays 6...e6, White instead focuses on playing an eventual f4 and achieving a Kingside attack.

White also often develops the bishop to c4 with 6. Bc4, known as the Lipnitsky attack. Usually Black responds with 6...e6, shutting down the bishop and increasing their control of the d5-square. White also usually castles kingside in this line, and Black usually fianchettoes the light squared bishop to b7.

Occasionally the move 6. Bd3 has also been tried, though this is less common than 6. Be2 and 6. Bc4 as the bishop on d3 is obstructed by the pawn on e4, and the bishop on d3 blocks the queen's support of the d5-square.

White can also try to accelerate his chances of a kingside attack by immediately pushing a Kingside pawn.

6. f4 is the Amsterdam Attack, in which White attacks the e5 square and creates a semi-open f-file. White usually hopes to attack via the f-file. White also usually castles kingside in this line, and also often plays a4 soon after to prevent b5.

6. f3 can be called the anti-Ng4 English Attack. This usually transposes to the standard English attack seen after 6. Be3, though with f3 inserted, White avoids the option of black playing 6...Ng4 after Be3.

6. g3 is the Zagreb Variation in which White fianchettoes the king's bishop to g2 in order to control the d5 square further, and discourage black from playing b5, as that could make the g2-bishop quite powerful.

6. h3 is the Adam's attack. In this variation, White controls the g4 square and prepares an eventual g4 in the future for an eventual Kingside attack. If Black stops this with h5 in the future, White usually follows up with g3 and Bg2, and plays for the d5 square.

6. h4 has also been tried, where White aims to try to accelerate his attack on the queenside faster. This is less common than the other kingside pawn moves, as it's quite a commital move, and Black usually equalizes comfortably.

White can also try to slow down the attack of Black's on the queenside by pushing a queenside pawn:

6. a4 prevents b5 entirely, therefore Black is unable to expand on the queenside further. However, it weakens the queenside and the b4 square, and therefore White usually doesn't castle queenside, instead castling kingside and focusing on controlling the d5 square, or playing for an eventual f4.

6. a3 prevents Black from playing b4 and kicking away the c3 knight in the future, though just like a4, also weakens the queenside, therefore if White castles queenside, this pawn could become a hook with an eventual b4. As a result, White usually castles kingside, and focuses on controlling the d5 square or playing for an eventual f4.

White can also play the surprising 6. Rg1!?, the Freak attack, aiming for an eventual g4 advance. This looks odd, but White intends to castle queenside and maintains flexibility with the kingside pawns, and can therefore decide the best places to advance them.

Another move often played is 6. Nb3, which anticipates the move e5 coming in the future, and therefore removes the knight from that position. This move discourages 6...e5 as a result, but also wastes a tempo, allowing Black to play a move like 6...g6, getting an improved Dragon Sicilian.

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

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6

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Wikipedia has related information at Sicilian Defence, Najdorf Variation

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