London System is a chess opening with the ECO code A48. It goes 1. d4 Nf6 2. Nf3 g6 3. Bf4. White opens with 1. d4 (the queen's pawn) and Black answers 1... Nf6. After 5 moves it is Black to move. The lesson on the board explains every move of the line and lets you play it yourself; it is free.
In chessGM's opening book this position continues with 3... Bg7.
chessGM has a full course for this line: open the London System course lesson.