Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e6/3. Nc3/3...Bb4

= Nimzo-Indian Defence =

3...Bb4
3...Bb4 introduces the Nimzo-Indian Defence. Black develops their dark-squared bishop as actively as possible and pins the white knight. If not handled properly, this pin can cause problems for White for several reasons. The most positionally significant of these is that the pin nullifies White's control of the e4 square. White would like to play e4 at some point, but Black's knight on f6 dominates the square for now, since the c3 knight cannot recapture when pinned. Consequently, Black may be able to install their knight on e4 or otherwise use the square for his own purposes if White does not fight for it.

A second implication of the pin is that Black is threatening to exchange bishop for knight and double White's pawns. The resulting position would be unbalanced, with White trying to open the position to utilize the power of his two bishops, and Black working to keep the position closed and exploit White's pawn weaknesses. White must decide how they feel about this prospect when choosing their next moves.

The Nimzo-Indian Defence is quite popular and there are a number of systems available to each side. White's fourth move options are listed below. Note that perhaps the most obvious move, 4. Bd2, breaking the pin and eliminating Black's threat to double White's pawns, is not among them. This is not because 4.Bd2 loses immediately; it does not. However, it is rarely seen in top level chess because it is far less ambitious than White's other choices.

Theory table
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'''1. d4 Nf6 2. c4 e6 3. Nc3 Bb4'''