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Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. e5/3...c5/4. c3
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French Defence: Advance Variation C02
4. c3
Here, after White's 4. c3, Black usually decides to increase the pressure on White's center pawns (d4/e5) by playing /4...Nc6/.
Another plausible plan for Black is /4...Qb6/, which also increases the pressure on d4, and allows the manoeuver 5...Bd7 followed by 6...Bb5. The idea is to trade Black's 'bad' bishop for White's 'good' bishop.
Theory table
Main line 4...Nc6 5. Nf3 Qb6 Wade variation 4...Qb6 5. Nf3 Bd7
References
French Defence