Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ng4/4. Nf3

Adler variation.
The Adler variation is named after the game Adler – Maróczy which was played at the 1896 Budapest tournament. White is ready to give up the e5-pawn in order to develop all their pieces on their best squares, i.e. the d5-square for the Nb1, the f3-square for the Ng1 and the a1–h8 diagonal for the Bc1.
In the main line 4...Bc5 the f2-pawn is attacked, forcing 5.e3 that blocks the way for the Bc1. Then after 5...Nc6 White has not enough pieces to protect their e5-pawn on the long run, e.g.:
An important theoretical decision for White is to choose whether they wants to make a2–a3 part of their plan or not. While this move avoids any possible Nc6–b4 and creates the possibility b2–b4, it may also be seen as a possible waste of time in some lines. As Lalic puts it:It was not so long ago that 8.a3, with the obvious intention of expanding with b2–b4, was the standard move. However, after Black responds with the logical a7–a5, it became apparent in tournament practice that the inclusion of these moves is in fact in Black's favour, as it gives their queen's rook access into play via the a6-square.
Black can also try the minor line /4...Nc6/ that allows Black to delay the development of the f8-diagonal depending on the circumstances, and allows White to transpose into the 4.Bf4 variation if they wish to do so.
The 4...Bc5 line with a2–a3.
"1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Bc5 5.e3 Nc6 6.a3"
After the standard moves 6...a5 7.b3 O-O 8.Bb2 Re8 9.Nc3 Ngxe5 10.Nxe5 Nxe5 11.Be2 d6 12.O-O both kings are in safety and Black has regained the invested pawn in the process. Both players can deviate at various points but the positions reached are similar, e.g. in one game White played an early Ra1–b1 in order to push b2–b4 in one move, but Black continued with the same plan as explained hereunder. In another game White developed their Bf1 in d3 instead of e2, but this gave the opportunity for Black to sacrifice the Ng4 on the f2-pawn before the white castle.
White has a space advantage in the center and can initiate pressure here or on the queenside by some pawn pushes like b3–b4 and c4–c5 (possibly supported by a knight on the d5-square). Meanwhile, the White king lacks some defenders so Black can start a pieces-driven attack with the standard "rook lift". As Tseitlin puts it, "the point is that 6...a5 fits into the plan of attacking White's kingside (!), whereas 6.a3 does little in the way of defending it". Thus if White does not find a clear way to make good use of their move a2–a3, it may turn out to be a waste of tempo.
After 12...Re6 White has to chose a way to react to the oncoming assault:
The 4...Bc5 line without a2–a3.
"1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Bc5 5.e3 Nc6"
In this variation White reasons that the advance a2–a3 is only a loss of time, because Black will play a7–a5 anyway. By refraining from the advance a2-a3 White tries to gain a tempo on the lines of the previous section, to make it more difficult for Black to initiate the Re8–e6–h6 lift. For example after the typical 6.Be2 O-O 7.O-O Re8 8.Nc3 Ngxe5 9.Nxe5 Nxe5 10.b3 d6 11.Bb2 Re6 12.g3 Rh6?! 13.Ne4 Qd7 14.h4 Black does not have the a7-square for their Bc5 because they had no time to play a7–a5. As we shall see, however, a rook lift is still one of the best options for Black.
White generally continues with 6.Be2, but can instead try the tricky 6.Nc3. The immediate recapture 6...Ngxe5?! 7.Nxe5 Nxe5 allows 8.f4! Nc6 9.Bd3 when the white bishop is much more actively placed than on e2. However, simply 6...O-O! 7.Be2 Ngxe5 transposes to the 6.Be2 move-order.
Note that Black should recapture the e5-pawn first with the Ng4, not the Nc6. Lalic analyzes 6.Be2 O-O 7.O-O Ncxe5?! 8.Nd4! (attacking the Ng4), when 8...Nf6 leaves Black without a clear plan and "White can expect an opening advantage". Alternatively, 8...Qh4 gives White a positional advantage with 9.h3 Nh6 10.Nc3 d6 11.Nd5 ("!" Lalic) to bring the knight back to f4 in case of a Black sacrifice in h3.
After the standard moves 6.Be2 O-O 7.O-O Re8 8.Nc3 Ngxe5 9.Nxe5 Nxe5 White has tried two different plans:
Instead of the usual 6.Be2 White has also tried developing the queenside first, but it is dangerous to leave White's king in the centre too long. After 6.b3 O-O 7.Bb2 Re8 8.Bd3 ("?!" Lalic) d6 ("!" Lalic) 9.exd6? Nxf2! 10.Kxf2 Rxe3 11.Kf1 Bg4 Black had a crushing attack.
White attacks in the centre.
"1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Bc5 5.e3 Nc6 6.Be2 O-O 7.O-O Re8 8.Nc3 Ngxe5 9.Nxe5 Nxe5 10.b3"
After 10.b3 a5 White can try to capture the Bc5 with 11.Na4 or 11.Ne4, one point being that the retreat 11...Ba7 would lock the Ra8 because Black has not played Ra8–a6 already. Nevertheless, Lalic still thinks 11...Ba7 is the right move after 11.Ne4 due to the importance of the a7–g1 diagonal. However, Black can also reroute the bishop with 11...Bf8 and "White has no obvious path to even a minute advantage". After 11.Na4 Black can also simply react by 11...b6 when the loss of the bishop pair is compensated by the semi-open B-file and the improved control on the central squares. Tseitlin considers that after the exchange in c5 Black has the better position. That is why the most common White move is 11.Bb2, keeping the knight jumps for later.
Then the most common plan for Black is a rook lift: the plan Ra8–a6–h6 was tried in the much-commented game Åkesson – Tagnon (Berlin Open 1984). Black duly won, but after the game continuation 11...Ra6 12.Qd5! Qe7 13.Ne4 Ba7 14.c5 Rg6 15.Rac1 Bb8 16.f4 authors do not agree on which side had the advantage. Borik considers that White had a positional advantage, Tseitlin agrees and recommends 15...Nc6! instead of 15...Bb8, with dangerous threats. However about 15...Bb8 Lalic says "it is true that the bishop pair look a bit pathetic lined up on the back rank just now, but there is no way to stop them breaking out later".
In fact, after 11...Ra6 much depends if White goes for the moves Qd1–d5 and Nc3–e4, or deviates with Nc3–a4:
White attacks on the kingside.
"1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Bc5 5.e3 Nc6 6.Be2 O-O 7.O-O Re8 8.Nc3 Ngxe5 9.Nxe5 Nxe5 10.Kh1"
The plan of attacking on the kingside was unveiled by Spassky in a game against Illescas (Linares 1990). Black did not understand immediately White's idea, so that after 10...a5?! 11.f4 Nc6 12.Bd3 d6 13.Qh5 (awarded a "!" by Lalic) 13...h6 14.Rf3 Black's pieces were ill-placed to counter White's attack.
A more principled plan for Black is to react by an action on the centre, specifically against the backward e3-pawn. After 10...d6 11.f4 Nd7 ("!" Lalic) 12.Bd3 Nf6 13.Qf3 Ng4 14.Nd1 f5 ("!" Lalic) and Black has succeeded in avoiding White's expansion e3–e4.
The 4...Nc6 line.
"1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Nc6"
Black plays this when they want to postpone the placement of its dark-squared bishop. Now White has a wide choice:
Theory table.
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3