King's Gambit Declined

Black declines the gambit the so called "Classical" way by developing his king's bishop to c5. On c5, it prevents white from castling without having to spend multiple tempos to shift the bishop or block its diagonal (for example, white will have to play Nc3 followed by Na4 or c3 followed by d4). Nevertheless, white gets a small advantage with correct play. The main replies to the Classical Variation include:

• A very important note: White cannot accept the pawn and play 3. fxe5, because of 3...Qh4+ 4. g3 leading to the loss of a rook (attempting to hang onto the rook with 4. Ke2 leads to 4...Qxe4#).
• 3. Nf3, is the most obvious option, preventing the queen from coming to h4 and readying fxe5. From here, black has multiple replies, including 3...d6 or 3...Nf6, from where, white can get ready to gain a tempo on the bishop via c3 then d4. 3...exf4 is unsound because of 4. d4!, pushing the bishop out. The queen can't come to h4, so white gets an advantage due to its imposing center on d4 and e4. Not that after 4. d4, black can't play 4...Bb4+, because 5. c3! only strengthens white's center AND gains a tempo on the bishop.
• 3. Bc4 looks good at first right, but is somewhat unsound after 3...d5! 4. Bxd5 Nf6 5. Nf3 Nxd5 6. exd5 e4 7. d4 Bb4+ 8. c3 exf3 9. cxb4 fxg2 10. Qe2+ Qe7 11. Qxe7+ Kxe7 12. Rg1 Bh3.