Accelerated London System (Mason Attack)

White develops the dark-square bishop. Developing the bishop first allows White to support their d4-pawn with e3, without trapping this bishop behind the pawn chain.

This is the beginning of the London system. The appeal of this system is that White can go for the same general set-up in response to most of Black's replies (see diagram). General plans for White include planting their kingside knight on e5, controlled by d4 and the dark-square bishop. Often White's light-square bishop is sacrificed on h7 to open up a castled king, followed by Qh5+.

Formally, the Accelerated London follows 1...d5, but the London set up can be played against 1...Nf6 just as well. It may transpose if Black plays d5, but by starting with Nf6, Black has the flexibility not to.

At higher levels, the move order 2. Nf3 Nf6 3. Bf4 to reach the London system is more common.

The main alternative for White is the Rapport-Jobava London system, with Nc3 (in either order, 2. Nc3 & 3. Bf4 or 2. Bf4 & 3. Nc3) where plans include the knight co-ordinating with the the f4 bishop to attack c7, and an early e4.

Because White can play the same system in response to most of Black's replies. White usually continues 3. e3, 4. c3, 5. Nd2 etc regardless. Most move orders are interchangeable.

Black's most common reply is 2...Nf6, then they have two main approaches: 3...c5 or 3...e6.

The idea behind the 3...e6 line is to play 4...Bd6, getting ready to castle quickly and threatening to trade off White's London bishop. White can drop their bishop back to g2, inviting Black to trade but after hxg2 White gets the semi-open h-file in return. In the e6 lines, Black's light square bishop often has difficulty developing. This approach resembles a Semi-Slav defence with colours reversed.

The ...c5 line (either 3...c5, or in the order 2...c5 3. e6 Nf6) is called the Steinitz countergambit. Black's idea is to prioritise Bf5 and Qb6 before they finish developing the kingside and castling. Black seeks counterplay by pressuring b2, which White left undefended when they moved their bishop early on. (White does not want to take the offered c5 pawn of course: they'd rather play c3 and let Black trade into them, so that they can keep two pawns in the centre).

Having committed to d5 already, the King's Indian-style approach with 3...g6 is less common but still playable. Black continues 4...Bg7, and 5...O-O. White usually includes h3, so there is a retreat square for their bishop on h2, and Be2.

Possibly the earliest example of 1. d4 d5 2. Bf4 was given by Jacob Sarratt in his 1808 book A Treatise on the Game of Chess. This continued 2...Bf5 3. Nc3 Nc6 4. f3.^[1]

James Mason pioneered the London system continuation (i.e. where White plays e3, places a knight on d2 and their light square bishop on d3 or e2) in the 1882 Vienna tournament.^[2] It became known as the London after the 1922 London tournament, where it was played by Capablanca^[3] and others.

The London system is popular with amateurs. It is often recommended to beginners as White can develop in the same way irrespective of how Black chooses to responses. It is occasionally seen at the highest levels of chess, e.g. in Ding v Nepomniachtchi in their 2023 World Championship match.^[4]

A recent innovation is to combine Bf4 and Nc3, the Rapport-Jobava London, developed by Richárd Rapport and Baduur Jobava.

For explanation of theory tables, see theory table and for notation, see algebraic notation..

1. d4 d5 2. Bf4

Wikipedia has related information at London System

• James Mason and the London System - Chessbase
• London System Opening Complete Guide 365Chess