The London System
The London System is a setup, not a long forced line. White develops the dark-squared bishop to f4 before closing the pawn chain with e3, then puts knights on f3 and d2, the light-squared bishop on d3, and castles kingside. You can play those moves against almost anything Black tries. The ideas stay the same.
The moves
A typical order against a solid Black center is 1.d4 d5 2.Nf3 Nf6 3.Bf4 e6 4.e3 Bd6 5.Bg3, or 5.Bxd6 if you prefer to take the bishop. Against an early ...c5, White plays c3 so the d4-pawn stays supported, then Nbd2 and Bd3.
The move order is flexible. Some players start 1.d4 and 2.Bf4 immediately. What you should not do is play e3 before the dark-squared bishop is outside the pawn chain. That bishop is the reason the system exists. Shutting it in on c1 leaves you with a passive Queen's Pawn Game and none of the London's pressure.
What White is doing
The bishop on f4 eyes the center and, from d3, the light-squared bishop eyes h7. The knight on d2 keeps the c-pawn free to play c3 or, later, c4. You have two real pawn breaks.
The first is e4, opening the center once your pieces are out and the king has castled. The second is c4, challenging Black's d5-pawn and opening the c-file for a rook. Pick one. Playing both at once usually drops a pawn. If Black has played ...c5 and put pressure on d4, c3 is the sturdy choice and e4 is the break you build toward.
What Black should do about it
The London bishop is the piece to challenge. ...Bd6 offers a trade of dark-squared bishops and takes the sting out of Bf4. ...Nh5 is another way to ask that bishop a question, at the cost of a move with the knight.
On the queenside, ...c5 and ...Qb6 hit d4 before White has finished developing. Black does not need a special refutation. Develop, castle, trade the f4-bishop if it is annoying, and meet e4 with a solid center. The system is reliable for White. It is not a forced advantage.
A mistake to avoid
Beginners fall in love with Bd3 and Qe2 or Qc2 aimed at h7, then sacrifice on h7 before the rest of the army has joined. A bishop and queen do not mate a castled king that still has a knight on f6. Build e4 or c4 first. The attack on h7 is a feature of the position, not a combination you play on move 9.
Other 1.d4 weapons, including the Queen's Gambit, are in nine openings worth knowing.
Frequently Asked Questions
Is the London good for beginners?
Yes. The piece placement repeats every game, so you spend your time on plans instead of on remembering a different line against every defense.
What is the Jobava London?
A sharper cousin with Nc3 early and often Bb5, attacking a knight on c6. It is a different system. Learn the classical London with Nbd2 and c3 before you add it.
Can Black play the London too?
Black can set up with ...d5, ...Bf5, and ...e6, but White's extra tempo makes an exact copy less comfortable. Black is usually better off challenging the center with ...c5.
One setup, two breaks
Get the bishop to f4, support d4, castle, and choose e4 or c4. Practice the setup against a person on Play vs AI, then review the game with chess hints to see whether the break was on time.
Try the line on the board with chess hints.