A simple and logical move, preparing to develop the bishop.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. g2g3  3... d7d5  4. f1g2  4... f8b4 [%c_arrow b1c3;keyPressed;none;from;b1;opacity;0.8;to;c3;persistent;false,c1d2;keyPressed;none;from;c1;opacity;0.8;to;d2;persistent;false,b1d2;keyPressed;none;from;b1;opacity;0.8;to;d2;persistent;false] A very popular choice in this particular move order of the Catalan Opening. This check forces White to transpose the game to a Nimzo-Indian after 5.Nc3, place the bishop in a slightly worse position with 5.Bd2, or place the knight in an undesirable square after 5.Nd2. 5. b1d2  5... e8g8  6. g1f3  6... b7b6  7. e1g1  7... c8b7 