Chess Opening Theory/1. d4/1...f5/2. Bg5/2...h6

= Dutch Defence with 2.Bg5 =

2. ... h6
Black drives back the white bishop. White can now play either Bf4 or Bh4, the latter maintaining the pin on the pawn. Because 2... h6 weakens black's kingside squares, some players prefer to avoid it, although there is no obvious refutation for white.

Theory table
1. d4 f5 2.Bg5 h6