King's Knight's Gambit

3. Nf3 develops the knight to its natural square, covers h4 against the queen check, and prepares d4, after which the c1-bishop can try to recover the pawn on f4. It is the most popular third move for White, ahead of the Bishop's Gambit, 3. Bc4. Because f4 is not attacked until White's d-pawn moves, Black has a free tempo, and the range of replies is unusually wide.

The most frequent answer is 3...g5, the Classical Variation, which supports f4 and threatens to drive the knight away with ...g4. White usually strikes at the chain with 4. h4, when 4...g4 5. Ne5 is the Kieseritzky Gambit, or plays 4. Bc4 with the Muzio Gambit, 4...g4 5. O-O, in mind. 4. Nc3, the Quaade Gambit, has been promoted by John Shaw as a less explored option. Two preparatory moves aim to keep the pawn while avoiding the Kieseritzky. The Becker Defense, 3...h6, builds a chain on h6, g5 and f4, so that after 4. d4 g5 5. h4 Bg7 the capture 6. hxg5 is met by 6...hxg5. The Fischer Defense, 3...d6, takes e5 from the knight; after 4. d4 g5 5. h4 g4 the knight must go back with 6. Ng1, leaving neither side with a developed piece.

Other replies use the spare tempo to counterattack. The Modern Defense, 3...d5, also called the Abbazia Defense, hits e4 and gives up the pawn for piece play; the main line runs 4. exd5 Nf6 5. Bb5+, and 5...c6 6. dxc6 bxc6 7. Bc4 Nd5 is the Botvinnik Variation. It can also arise from the Falkbeer Countergambit, and White's extra central pawn is thought to give a slight edge. The Schallopp Defense, 3...Nf6, answers 4. e5 with 4...Nh5, guarding f4 from the rim. The MacLeod Defense, 3...Nc6, is usually followed by 4. d4 d5 5. exd5 Qxd5 6. Nc3 Bb4. The Cunningham Defense, 3...Be7, renews the threat of ...Bh4+ to keep White's king in the center; the standard reply 4. Bc4 means that 4...Bh4+ can be met by 5. Kf1. The Bonsch-Osmolovsky Variation, 3...Ne7, prepares ...d5 so that a knight can recapture there and protect f4. The Gianutio Countergambit, 3...f5, and the Wagenbach Defense, 3...h5, are rare.

Engines rate the position as slightly better for Black and prefer 3...d6, 3...h6 and 3...Nf6 to the immediate 3...g5. In online practice White nonetheless scores clearly better overall, with the MacLeod a particularly favorable reply for White. Among the commonly played defenses, only the Cunningham and the Bonsch-Osmolovsky show Black scoring ahead.