A useful, and flexible move.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... d7d5  4. c4d5  4... e6d5  5. c1g5 World championship matches give a popularity boost to the lines that were explored there. 5... c7c6  6. e2e3  6... h7h6  7. g5h4  7... f8e7  8. f1d3  8... e8g8  9. d1c2  9... f8e8 The Carlsbad pawn structure has been reached, one of the most solid setups. 10. g1e2 