The Slav Defense begins with c6 against c4. Its defining strategic idea is to support the d5-pawn without blocking the c8-bishop, which is what ... e6 does in the Queen's Gambit Declined.
1. d4 d5 2. c4 c6
The pawn chain c6-d5 is the same one Black plays in the Caro-Kann (here), and the strategic ideas carry over:
The usual move order is:
1. d4 d5 2. c4 c6 3. ♘f3 ♞f6 4. ♘c3 dxc4
Black gives up the central pawn for a moment, but White cannot keep the extra material. White recovers the pawn on c4, and in the meantime Black develops the bishop.
White's most testing continuation is to prevent ... b5, which would allow Black to hold the extra pawn:
1. d4 d5 2. c4 c6 3. ♘f3 ♞f6 4. ♘c3 dxc4 5. a4 ♝f5 6. e3 e6 7. ♗xc4 ♝b4 8. O-O ♞bd7 9. ♕e2 ♝g6 10. e4 O-O 11. ♗d3 ♝h5
The move a4 stops ... b5, but it also has a price: the a-pawn can no longer play a3, so the bishop on b4 cannot be targeted by a pawn. After ... ♝b4 the knight on c3 is pinned against the king, and White has to spend a move on castling before the center can be opened.
The key recurring ideas in this line are:
After the same first moves, White can play the knight to e5 instead of e3. This is the sharper way to fight for the center:
1. d4 d5 2. c4 c6 3. ♘f3 ♞f6 4. ♘c3 dxc4 5. a4 ♝f5 6. ♘e5 ♞bd7 7. ♘xc4 ♛c7 8. g3 e5 9. dxe5 ♞xe5 10. ♗f4 ♞fd7 11. ♗g2 ♜d8
Black prepares ... e5 with ... ♛c7 and then challenges the center directly. The resulting positions are more tactical than the e3 lines, and the best moves depend on concrete details. This is the part where the theory is heaviest, so I would study it after the main line is comfortable.
If White plays e3 instead of a4, Black can keep the pawn with ... b5:
1. d4 d5 2. c4 c6 3. ♘f3 ♞f6 4. ♘c3 dxc4 5. e3 b5 6. a4 b4 7. ♘a2 e6
The pawn on b4 attacks the knight and drives it to the edge of the board. The fight in this line revolves around the queenside: White wants to recover the c4-pawn and pressure b4, while Black wants to hold the extra pawn and develop the pieces smoothly. It is a concrete line, so the exact move order matters.
The Slow Slav starts with e3 before the knight comes to c3. White does not take the pawn on c4 immediately, and Black does not take it either:
1. d4 d5 2. c4 c6 3. ♘f3 ♞f6 4. e3 ♝f5 5. ♘c3 e6 6. ♘h4 ♝g6 7. ♘xg6 hxg6
The knight on h4 attacks the bishop, so Black retreats and accepts the exchange. White obtains the bishop pair, but Black has a very solid structure and an open h-file. Here the bishop has been developed before ... e6, exactly as in the Caro-Kann, and the exchange does not damage the pawn chain.
White can also resolve the central tension immediately:
1. d4 d5 2. c4 c6 3. cxd5 cxd5 4. ♘c3 ♞f6 5. ♗f4 ♞c6 6. e3 ♝f5
The pawn structure is symmetrical, and the c-file is open for both sides. Black develops with ... ♞c6 and ... ♝f5, prepares ... e6, and places a rook on the c-file. The Exchange Slav is one of the calmest ways to meet the Slav, but the position is not equal by definition: a small lead in development or a more active queen can still create pressure, so both sides have to play accurately.
If White does not play c4, there is no Slav: ... c6 is the answer to the c-pawn, and the structure only arises when White has committed to both d4 and c4. Against the London, the Colle, and the Torre, ... c6 is playable but passive, and the more active plan is ... c5, followed by ... ♞c6 and ... ♛b6 or ... ♝f5 depending on White's setup. The bishop on c1 has left the queenside in the London, so b2 is the natural target. The detailed structures are discussed in the London page here.
If White later plays c4, for example after 2. ♘f3 ♞f6 3. c4, Black can return to the Slav with ... c6.
This page does not treat the Semi-Slav (... e6 combined with ... ♞bd7) or the other sidelines that White can choose at move three and four. The focus is the classical Slav with ... dxc4 and the early development of the c8-bishop, which is the direct counterpart of the Caro-Kann structures.