The Catalan is a popular 1. d4 opening, which can occur from the move order 1. d4; 1. d5; 2. c4; 2. e6; 3. g6; 3. Nf6 and many others. This is a common trap that can occur in the Catalan, which basically gains a free minor piece, and ruins your opponent's pawn structure. This is easy to fall into, so be careful when you play!