You'll recall that it's White's fifth move that will largely define the variation of the KI that will be played. The Classical is marked by 5.Nf3. This forecloses the other major KI variations. The Knight at f3 blocks the f-pawn, which is required to be pushed in both the Four Pawns (5.f4) and Saemisch variations (5.f3), and the Averbakh (5.Bg5) is foreclosed more indirectly, as Nf3 doesn't usually appear in the early moves of the Averbakh.mainlines. In the Classical, the Knight develops to f3 naturally and supports the d4 and e5 squares. The notion here is to review in a straightforward way the gist of the play, which you can add to with your own research, whether as White or Black. Separate posts will be dedicated to the major sublines of the Classical, including the Mar del Plata, Petrosian, Old Mainline, Na6, Gligorich System, and Exchange.
After 5.Nf3, Black invariably plays 5...O-O. Note that Black can't play e5 yet, as his game's a mess after 5...e5 6.dxe5 dxe5 7.Qxd8+ Kxd8 8.Nxe5 Nxe4 9.Nxe4 Bxe5 10.Bg5+ Ke8 11.Nf6+ Bxf6 12.Bxf6 Rg8 13.0–0–0. After 5...O-O, the major White response is 6.Be2, with 6.h3 serving as a fashionable minor alternative. Following 6.h3, Black continues 6...e5 (6...c5 is a minor response) 7.d5 (7.dxe5 is sometimes played with dxe5 following and now White has 8.Qxd8, 8.Be3, or 8.Bg5) and now 7...Na6!? (7...a5) with play potentially continuing 8.Bg5 h6 9.Be3 Nc5 (White will be slow to exchange the dark-squared Bishop for this Knight). Now back to the mainline. After 6.Be2, Black usually replies 6...e5, but Black obviously has other options, including 6...Na6 (see G116) and White has the major response of 7.O-O and minor responses of 7.Be3 (Gligorich System), 7.d5 (Petrosian System), and 7.dxe5 (Exchange Variation). Black next plays 7...Nc6 to reach the diagrammed position (right diagram). If Black plays 7...Nbd7, then it's the Old Mainline. If Black rather plays 7...exd4, then it's sometimes called the Glek Variation. After 7...Nc6, Black should anticipate 8.d5 Ne7 (often referred to as the Mar del Plata Variation), and now White has three often played moves, which will be the subject of separate posts: 9.b4 (the Bayonet), 9.Ne1, and 9.Nd2 (see Game 39 here, and Game 117 here) (diagram below).