Here's a chess question someone asked me. ( I can't answer it and I refuse to join a Chess forum just to find the answer.)
After the moves 1.d4 Nf6 2.c4 e6 if White plays 3.Nf3 then (in master level play) Black often plays the Queen's Indian Defense with 3...b6. But if White plays 3.Nc3 then Black rarely plays 3...b6. Why?