The Mason Variation is a variation of the London System, catalogued as ECO A48. The line runs 1. d4 d5 2. Bf4 Nf6 3. e3 e6 4. Nd2 c5 5. c3 Nc6 6. Ngf3 Bd6 7. Bg3, reaching the position shown below after 13 half-moves.
Both sides have pawns in the centre (White on d4, Black on d5), and one pawn capture is available, so the structure has not settled yet. Development is level at three minor pieces each. It is Black to move after seven moves.
On the London System generally: A solid system for White with d4, Nf3, Bf4. Easy to learn with consistent plans.
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The Mason Variation goes 1. d4 d5 2. Bf4 Nf6 3. e3 e6 4. Nd2 c5 5. c3 Nc6 6. Ngf3 Bd6 7. Bg3. It is a variation of the London System and is catalogued as ECO A48.
The Mason Variation is ECO A48. ECO codes group openings by their move order, so nearby codes cover closely related lines of the London System.
Stockfish evaluates the position after 1. d4 d5 2. Bf4 Nf6 3. e3 e6 4. Nd2 c5 5. c3 Nc6 6. Ngf3 Bd6 7. Bg3 at +0.18 at depth 18, which is balanced. Engine evaluation is only a starting point — at club level the practical value of a line is mostly about whether you know its plans better than your opponent does.
Both sides have pawns in the centre (White on d4, Black on d5), and one pawn capture is available, so the structure has not settled yet. Development is level at three minor pieces each. It is Black to move.