1 e4 c6 2 d4 d5 3 exd5 cxd5 4 c4
This is the so-called Panov variation. It is one of the most agressive lines against the Caro-Kann, and regarded by many as the critical test of the defence. It is also perhaps the most "hybrid" of variations, not only of the Caro-Kann, but of all opening theory, since it can branch off into the most diverse openings (Scandinavian, Grundfeld, Nimzo-Indian, Sicilian Defence, English opening). However, it is most frequently reminiscent of the Queen's gambit.
4 c4 was played by Leonhart as long ago as the last last century, but it was Alekhine and Botvinnik who, by elaborating on Panov's groundwork, eventually developed it into a true system. Indeed, the variation is also commonly known as the Panov-Botvinnik attack.
4 ... Nf6
The most natural answer and the most common in practice: ... dxc4 or ... e6 can be played later, so Black avoids committing himself immediately. The only deviation of real significance here is 4 ... Nc6!? after which White can transpose to other lines by way of 5 Nf3 Bg4 6 Nc3 Nf6, while the continuation 5 Nc3 e5! gives Black acceptable play according to Schwartz: 6 cxd5! Nxd4 7 f4 f6 8 Nf3 +=
Stronger, however, is 5 cxd5! Qxd5 6 Nf3 transposing to a minor variation of the Sicilian defence that favours White.
5 Nc3 ...
5 c5!? g6 6 Nc3 Bg7 7 Bb5+ Nc6 8 Nge2 0-0 9 0-0 Mariotti-Savon, Venice 1974, 9 ... Bd7 ∞. This position after 5 Nc3 is the basic starting point of the Panov. The most frequent continuations are now 5 ... Nc6 and 5 ... e6 and may be analyzed in subsequent nodes.
5 ... g6
The bishop's development on the flank is logical, but a bit slow. In most instances, Black has to sacrifice his d5 pawn, at least temporarily, in order to complete his development. The most popular continuations for White at this point are 6 cxd5 and 6 Qb3.
A couple of rarer variants include:
6 Nf3 Bg7 7 Qb3 dxc4 8 Bxc4 0-0 9 Ne5 e6 10 Be3 Nc6 11 Nxc6 bxc6 12 0-0 Qc7 13 h3 Nd5 14 Rac1 Rb8 15 Qc2 Nxe3 16 fxe3 c5! =+ Pgats-Szabo, Budapest 1961.
6 Bg5 Bg7! 7 Nf3 Ne4 8 cxd5 Nxg5 9 Nxg5 0-0 10 Bc4 e5 11 Nf3 exd4 12 Nxd4 Qh4 14 Nce2 Bg4 = Bessenay-Maric, France 1971.
Explanation of chess notation used in this writeup.