London System: one setup against everything
The London System — 1.d4 followed by Bf4, e3, Nf3, c3, Bd3, Nbd2, and castling short — is the rare opening you can play against almost anything Black tries. Instead of memorizing branching theory, you learn one pawn-and-piece structure and the plans that flow from it. That's why it has become the most popular opening at every level from club night to the world elite.
The move that defines the London is Bf4: the queen's bishop gets outside the pawn chain before you play e3, solving the problem that haunts the Queen's Gambit player. The resulting pyramid of pawns on c3, d4, and e3 is extraordinarily hard to attack, and every piece has a job: the d3 bishop looks at h7, the f3 knight watches e5, and the d2 knight is ready to jump to e5 or support a c4 break.
Your main attacking plan is simple to say and dangerous to face: plant a knight on e5, support it, and start pushing kingside pawns or lifting a rook. Against a fianchetto setup like the King's Indian, you'll often switch to a queenside plan with c4 and expansion. The London isn't passive — it's patient. You build the same safe house every game and then attack from it.
Black's best tries aim at your bishop or the d4 pawn: an early ...c5 with ...Qb6 hitting b2, or ...Bd6 offering to trade the f4 bishop. Learn the standard answers — Qb3 or a timely dxc5 against the queenside pressure, and meeting ...Bd6 with Bg3 so a trade on g3 opens the h-file toward Black's king rather than fixing your structure.
The honest trade-off: the London gives you fewer chances for an early knockout than 1.e4 openings, and well-prepared opponents can equalize. What it gives you instead is a game you understand better than your opponent from move two. For players who want to spend their study time on plans and tactics rather than theory, that trade is a bargain.