The Semi-Slav 1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 e6, 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Nc3 c6, or 1. d4 d5 2. c4 c6 3. Nf3 e6 4. Nc3 Nf6. The Semi-Slav does not arise until Black has made his fourth move.
The Mainline Slav 1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 dxc4. The Slav also gets most of the lines that occure after 1. d4 d5 2. c4 c6, which includes the a6 systems.
The Cambridge Springs variation is also hard to pigionhole. 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Nbd7 5. Nf3 c6 6. e3 Qa5 or 1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 e6 5. Bg5 Nbd7 6. e3 Qa5. It is just as much a Semi-Slav as it is a QGD.
When does the Slav and when does the SemiSlav occur?At Wikipedia it says that SemiSlav is QGD variation(e6) but in many Slav games I see the e6 move being played in the opening...so I am kinda confused...I would really appreciate any help about this mixup and if someone could point out the difference between the two(mainly around the e6 move)Thanks in advance!