5...c4: the closed queenside is variation 2 of 6 in the Colle System course, a beginner opening course for White (ECO D05). It runs 23 moves and begins 1. d4 d5 2. Nf3 Nf6 3. e3 e6 4. Bd3 c5 …. Black closes the queenside with ...c4 and chases the bishop. White keeps the bishop on the h7 diagonal and answers with the e4 break. In the lesson every move is explained on a board and then drilled; it is free.
Move by move
1. d4 – White takes the centre with the queen's pawn.
1... d5 – Black meets it in the centre.
2. Nf3 – The knight develops and guards d4 and e5.
2... Nf6 – Black develops too.
3. e3 – The Colle move. It opens the f1 bishop and keeps d4 solid.
3... e6 – Black opens the way for the dark bishop.
4. Bd3 – The bishop goes to d3 and aims at h7.
4... c5 – Black hits d4 from the side.
5. c3 – The Colle triangle. White prepares the push to e4.
5... c4 – Black closes the queenside and attacks the d3 bishop.
6. Bc2 – The bishop steps back and keeps aiming at h7.
6... Nc6 – Black develops.
7. Nbd2 – The knight supports the e4 push. That push is White's answer to the closed queenside.
7... Bd6 – Black's bishop aims at h2.
8. O-O – White castles.
8... O-O – Black castles too.
The lesson goes on for 7 more moves, each with its explanation.
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