A rarer third option is 4 ... Bb4 +, which aims to exchange the less useful dark-squared bishop, though this line tends to leave Black with a slightly passive position.

In the Rubinstein variation 3 ... Ng4 4. Bf4 with 4 ... Nc6 5. Nf3 Bb4 + 6. Nbd2 Qe7 7. a3, after 7 ... Ngxe5 8. Nxe5 Nxe5 9. e3 Bxd2 + 10. Qxd2 ( see diagram at right ) White gets the bishop pair and a space advantage.

Similarly, in the Rubinstein variation 3 ... Ng4 4. Bf4 with 4 ... Nc6 5. Nf3 Bb4 + 6. Nc3, after 6 ... Bxc3 + 7. bxc3 White is saddled with doubled pawns in c3 and c4 that limit the scope of his bishop pair.

Lalic thinks the strategies in which Black gives up the bishop pair ( by exchanging its Bb4 for the Nd2 ) for nothing are a mistake.

The move 11. Nb3 is not only useful to support the c4 – c5 push, but also to exchange the knight against Black's dark-squared bishop after a possible a2 – a3 forcing the retreat Bb4 – c5.

The Queen's Indian Defence also has a variation developed by Petrosian: 1. d4 Nf6 2. c4 e6 3. Nf3 b6 4. a3, with the idea of preventing ... Bb4 +.

White often plays 3. g3 or 3. Nf3 to avoid the Nimzo-Indian, allowing him to meet 3. Nf3 Bb4 + ( the Bogo-Indian Defence ) with 4. Bd2 or 4. Nbd2, rather than 4. Nc3.

A topical line would go 1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Qc2 Nc6 5. Nf3 ( better than 5. e3 ) d6, with the idea of playing e5.

This line can also arise from the Bogo-Indian Defence ( 1. d4 Nf6 2. c4 e6 3. Nf3 Bb4 +) if White blocks the check with 4. Nc3.

All codes begin with 1. d4 Nf6 2. c4 e6 3. Nc3 Bb4, excluding E20, which also includes alternatives to 3 ... Bb4 apart from 3 ... d5 ( which would be the Queen's Gambit Declined ).

Most often these defences consist of ... Nf6, ... e6, and ... d5 or ... Bb4 systemic responses by Black, or a Slav-like system consisting of ... c6 and ... d5, a direct King's Indian Defense setup with ... Nf6, ... g6, ... Bg7, ... 0-0, after which ... c5 and ... e5 are eschewed, or 1 ... f5, which usually transposes to a Dutch Defense once White plays d4.

First, the " Reversed Ruy Lopez " with 3 ... Bb4 is ruled out.

After 3 ... e6, White can play 4. Nc3 Bb4 ( transposing to the Nimzo-Indian Defense ); 4. a3, when Black can either play 4 ... d5 ( reaching a kind of Queen's Gambit Declined or Ragozin System ), or 4 ... d6 preparing 5 ... e5 or even 5 ... g6 (" championed by Bologan ", according to Palliser ), reaching a sort of King's Indian Defense ; or 4. g3, when Black can transpose to the Catalan Opening with 4 ... d5, recommended by Palliser or 4 ... Bb4 +, preferred by Orlov, which transposes to a Nimzo-Indian after 5. Nc3, or to a Bogo-Indian Defense after 5. Bd2 or 5. Nbd2.

Krasenkow's other important contribution is the so-called Groningen Attack in the English Opening ( discovered simultaneously with Vadim Zviagintsev ): 1. Nf3 Nf6 2. c4 e6 3. Nc3 Bb4 4. g4!

After 3. dxe5 Ng4, there is a battle over White's extra pawn on e5, which Black typically attacks with ... Nc6 and ( after ... Bc5 or ... Bb4 +) ... Qe7, while White often defends it with Bf4, Nf3, and sometimes Qd5.

Rubinstein showed how White could get a small positional advantage with 4. Bf4 Nc6 5. Nf3 Bb4 + 6. Nbd2, an assessment still valid today.

It occurs in the Rubinstein variation 3 ... Ng4 4. Bf4 with 4 ... Nc6 5. Nf3 Bb4 + 6. Nbd2 Qe7 7. a3.

The Bb4 is attacked but Black does not have to move it for the moment, and instead both regains the gambit pawn and sets a trap with 7 ... Ngxe5 ( see diagram at right ).

Apart from the sideline 4 ... g5, the main line continues with both players developing their pieces around the e5-pawn with 4 ... Nc6 5. Nf3 Bb4 + when White has an important choice between the moves 6. Nc3 and 6. Nbd2, each leading to extremely different play.

Black does best to immediately exchange the Nc3 with 6 ... Bxc3 + 7. bxc3 as otherwise White gets a small positional advantage simply by avoiding the doubled pawns ( see the section " Advantages of ... Bb4 +").

Against the mundane 10 ... d6 White can continue with 11. Nb3 ( see diagram at right ) to play on the queenside against the exposed Bb4, or 11. Nb1 to recycle the knight on the ideal d5-square.

: E21-1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Nf3 ( Kasparov Variation excluding 4 ... c5 5. g3, which is covered under E20 )

6 ... Bb4 + 7.

6. Nb3 and 6. Nf3 can be well met by 6 ... Bb4, threatening to win White's pawn on e4.

6. Nde2 can be met by either 6 ... Bc5 or 6 ... Bb4.

If Black is not aiming for the Sveshnikov, the main alternative is to play 6 ... Bb4 in reply to 6. Ndb5.

The idea is that 3. d4 runs into 3 ... cxd4 4. Nxd4 Nf6 5. Nc3 e5, when Nb5 is prevented, and Black will equalize by playing ... Bb4 and ... d5 at some point.

After 3. Nf3, Black usually plays 3 ... b6 ( the Queen's Indian Defence ) or 3 ... d5 ( leading to the Queen's Gambit Declined ), but can instead play 3 ... Bb4 +, the Bogo-Indian, named after Efim Bogoljubov.

If Black does not wish to play the Queen's Indian in response to 3. Nf3, alternatives include: 3 ... d5, transposing to the Queen's Gambit Declined ; 3 ... Bb4 +, the Bogo-Indian Defense ; and 3 ... c5, which typically leads to a Modern Benoni or a Symmetrical English.

However, it weakens the dark squares slightly, which Black can take advantage of by playing 5 ... Bb4 +.

After 5. b3, Black also has several playable alternatives to 5 ... Bb4 +, the most common of which is 5 ... Bb7 6.

Although Pirc had a minus record against Alexander Alekhine, he beat Alekhine with the black pieces in a blitz game in Ljubljana in 1930: 1. d4 e6 2. c4 Nf6 3. Nc3 Bb4 4. Bd2 b6 5. f3 Bxc3 6. Bxc3 d5 7. e3 0-0 8. Bd3 c5 9. Ne2 Nc6 10. 0-0 Bb7 11. Qa4 Qd7 12. Qc2 Nb4 13. Bxb4 cxb4 14. b3 Rac8 15. e4 h6 16. e5 dxc4 17. bxc4 Nd5 18. Qd2 Nc3 19. Rae1 Rfd8 20. d5 exd5 21. c5 Rxc5 22. Nd4 Bc8 23. e6 Qc7 24. exf7 + Kxf7 25. f4 Ne4 26. Qb2 Rc3 27. Nf3 Kg8 28. Ne5 Qc5 + 29. Kh1 Qd4 30. Qe2 Bf5 31. g4 Ng3 + 32. hxg3 Bxd3 33. Nxd3 Rxd3 34. Rd1 Qe4 + 35. Qg2 Rc8 36. Rxd3 Qxd3 37. Rf2 Rc1 + 38. Kh2 a5 39. Rd2 Qe4 40. Qxe4 dxe4 41. Kg2 a4 42. Rd4 Rc2 + 43. Kf1 Rxa2 44. Rxb4 e3 45. Rxb6 e2 + 46. Kf2 a3 47. Ra6 Ra1 48. Kxe2 a2 0 – 1

After the standard moves 7 ... Ngxe5 8. Nxe5 Nxe5 9. Be2 followed by 10. 0-0 it is Black's last chance to exchange the Bb4 for the Nd2.

Borik thinks 4. Qc2 is the move " that gives Black the most problems to solve ", but Lalic does not agree at all, stating that the reply " 4 ... Bb4 + followed by d7 – d5 ensures Black a rapid development and plenty of counterplay.

Today, the line arises quite often by transposition from the Black Knights Tango, e. g. 1. d4 Nf6 2. c4 Nc6 3. Nf3 e6 4. Nc3 Bb4 5. Qc2.

The Petrosian Variation, prepares 5. Nc3 without being harassed by ... Bb4 pinning the knight.

In the following game, using the " hedgehog " formation he in part originated ( characterized by a half-open c-file and pawns on a6, b6, d6, and e6, knights on d7 and f6, bishops on b7 and e7, a rook on c8, and kingside castling ), Andersson inflicts upon Anatoly Karpov the latter's first loss as world champion: Karpov – Andersson, Milan 1975 1. e4 c5 2. Nf3 e6 3. d4 cxd4 4. Nxd4 Nc6 5. Nb5 d6 6. c4 Nf6 7. N1c3 a6 8. Na3 Be7 9. Be2 0-0 10. 0-0 b6 11. Be3 Bb7 12. Rc1 Re8 13. Qb3 Nd7 14. Rfd1 Rc8 15. Rd2 Qc7 16. Qd1 Qb8 17. f3 Ba8 18. Qf1 Nce5 19. Nab1 Nf6 20. Kh1 h6 21. Rdd1 Bf8 22. Nd2 Rcd8 23. Qf2 Ned7 24. a3 d5 25. cxd5 exd5 26. exd5 Bd6 27. Nf1 Rxe3 ( an exchange sacrifice ) 28. Nxe3 Bxh2 29. Nf1 Bf4 30. Rc2 b5 31. Bd3 Nb6 32. Be4 Nc4 33. a4 Re8 34. axb5 axb5 35. Re2 Be5 36. Qc5 Nd6 37. Na2 Ndxe4 38. fxe4 Bd6 39. Qc2 Re5 40. g3 Qe8 41. Rde1 Bb7 42. Kg1 Nh7 43. Nc1 Ng5 44. Nd2 Bb4 45. Kf2 Bxd2 46. Rxd2 Nxe4 + 47. Rxe4 Rxe4 48. Ne2 Bc8 49. Nc3 Re1 50. Ne2 Ra1 51. Rd4 Qd8 52. Qc6 Bd7 53. Qd6 Qe8 54. Qf4 Qc8 55. b4 Bh3 56. Qe4 Bf5 57. Qe3 Qc2 58. g4 Bd7 59. Qe4 Qb3 60. Qd3 Qb2 61. Qe4 Ra8 62. Qe3 Ra2 63. d6 Ra8 64. Re4 Bc6 65. Qd4 Qb1 66. Re7 Qh1 67. Qf4 Qg2 + 68. Ke1 Ra1 + 69. Kd2 Qd5 + 70. Qd4 Ra2 + 71. Kc3 Qf3 + 72. Re3 Ra3 + 73. Kd2 Ra2 + 74. Ke1 Qh1 + 75. Kf2 Qg2 + 76. Ke1 Qh1 + 77. Kf2 Ra1 78. Rc3 Qg2 + 79. Ke3 Qf3 + 0 – 1

After 4. dxe5, Bauer concludes that " White stands a trifle better " but that " provided he plays accurately, Black doesn't have much to fear following 6. Bc4, by choosing any of the three valid replies, 6 ... Ke8, 6 ... Bb4, or 6 ... Be6.

Then the typical 4 ... Ng4 may be met by 5. d4 exd4 6. Na4, when 6 ... Bb4 +, White's usual move in the mirror-image position, is impossible.

The first one, resurrected and elaborated by the grandmaster Pavel Blatny, is to exchange the Bb4 for the Bf4.

After 4 ... Bb4 + 5. Nbd2 Nc6 6. a3 Black can easily get confused by the move-order.

Keilhack writes that this variation " occurs rather often and offers excellent chances for an early knockout " by White and that " only two ( 5 ... Bb4 and 5 ... Bc5 ) do not immediately ruin Black's game "