London System

A system more than a line: White develops the bishop to f4, plays e3 and c3, and sets up the same structure against almost anything. It is named after the London tournament of 1922 and is played from club level all the way up to Magnus Carlsen.

It is reached after 5 moves: 1. d4 Nf6 2. Nf3 g6 3. Bf4.

Black is to move and has 23 legal moves. Over the next four moves — two for each side — the game can branch into 678,246 different games. Those games reach 225,664 different positions: most positions can be reached in more than one way, 3 on average.

The Lichess opening list names 6 further lines after this position.

The same moves can be played in 6 different legal orders that all reach this position. For example 1. Nf3 Nf6 2. d4 g6 3. Bf4, which starts as the Zukertort Opening.