The Main Line is a variation of the Colle System, catalogued as ECO A04. The line runs 1. d4 d5 2. Nf3 Nf6 3. e3 e6 4. Bd3 c5 5. c3, reaching the position shown below after 9 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 two minor pieces each. It is Black to move after five moves.
On the Colle System generally: ECO A04. A chess opening with 3 known variations.
You can play through this variation move by move in your browser — Learn mode walks the line with an explanation at each step, and Drill mode replays it from memory so you find out whether you actually know it. No account is required.
See all 6 Colle System variations →
The Main Line goes 1. d4 d5 2. Nf3 Nf6 3. e3 e6 4. Bd3 c5 5. c3. It is a variation of the Colle System and is catalogued as ECO A04.
The Main Line is ECO A04. ECO codes group openings by their move order, so nearby codes cover closely related lines of the Colle System.
Stockfish evaluates the position after 1. d4 d5 2. Nf3 Nf6 3. e3 e6 4. Bd3 c5 5. c3 at -0.08 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 two minor pieces each. It is Black to move.