Chess Opening Theory/1. d4/1...Nf6/2. Nc3/2...d5/3. Bf4

Sometimes referred to as the Rapport-Jobava System. This opening serves as a good surprise weapon against unprepared opponents, and in the worst case, equalizes. White often plans to threaten a knight fork on the c7 square after Nb5, usually played on the fourth or fifth move. Black typically challenges the center immediately with 3...c5 and trades their c-pawn for White's e-pawn. Alternatively, Black often plays the 3...a6 Anti-Jobava to control the b5 square or the natural 3...e6 development move. Black may also attempt 3...Bf5, developing the light-squared bishop before playing 4...e6, but it gives White an opportunity to launch a pawn storm with a tempo on the bishop through a well-timed g4 push (usually after White plays f3 or Ne5, protecting the g4 square).

1. d4 Nf6 2. Nc3 d5 3. Bf4

Wikipedia has related information at London_System.