QGD :: should Black fiancheto his King's Bishop? A common line in the orthodox line of the QGD goes something like this:



[FEN ""]
1. d4 d5 2. c4 e6 3. Nf3 Nf6 4. Nc3



Here's the part that puzzles me. According to the database in 365chess Black's most common response in 4...Be7 presumably to support the knight in case of 5.Bg5 (which remarkably according to the same database is White's most common move even though after 4...Be7 no tempo is won). Anyhow, why doesn't Black simply play 4...g6 instead so he can conveniently fianchetto the King's bishop in response to 5.Bg5 ? This move does appear in the aforementioned database but way, way low. Why doesn't Black try to occupy the long diagonal with his King's bishop? Is there any strategic reason for Black in QGD for the King's bishop to remain on the a3-f8 diagonal instead?
The main reason why Black tends not to fianchetto in this position is because she has already played ...e6, which in conjunction with playing ...g6 to develop the bishop, unnecessarily weakens the dark squares on the black kingside without gaining anything; instead ...e6 has wasted a tempo that could otherwise be saved, and potentially hindered development of the other bishop. In other words, if Black is about to spend a move playing ...g6 to develop the kingside, it is better to wait on spending a move on the e-pawn.

Now it's true that, having played 1...d5 to start, the move ...e6 serves a purpose of reinforcing the pawn on d5. But this comes at the costs described above. That's why, if Black wants to fianchetto, she instead usually avoids an immediate 1...d5 and opts for things like the Gruenfeld or the King's Indian, moving straight ahead with that scheme of development, and pushing one way or another in the center after other things are clearer.

Consider a mainline King's Indian:



    [FEN ""]

1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.Nf3 O-O 6.Be2 e5




Note that Black has here gotten in ...e5 in one fell swoop. And in a Gruenfeld, 1.d4 Nf6 2.c4 g6 3.Nc3 d5, Black has played ...d5 but intends to get away without a reinforcing ...e6 move, instead playing for piece play against White's center.