The King's Gambit is a chess opening that begins with the moves: 1. e4 e5 2. f4
White offers a pawn to divert the Black e-pawn so as to build a strong centre with d2–d4. Theory has shown that in order for Black to maintain the gambit pawn, he may well be forced to weaken his kingside.
The King's Gambit is one of the oldest documented openings, as it was examined by the 17th century Italian chess player Giulio Polerio. It is also in an older book by  Luis Ramirez de Lucena. 
The King's Gambit is infrequently seen at master level today, as Black can obtain a reasonable position by returning the extra pawn to consolidate. There are two main branches, depending on whether or not Black plays 2...exf4: the King's Gambit Accepted (KGA) and the King's Gambit Declined (KGD)
King's Gambit Accepted
1. e4 e5
Black usually accepts with 2...exf4 .
White then has two main continuations:
a) 3.Nf3, the King's Knight Gambit is the most common as it develops the knight and prevents 3...Qh4+;
b ) 3.Bc4, the Bishop's Gambit , where White's development will rapidly increase after the continuation often played in the 19th century, 3...Qh4+!? 4.Kf1 followed by 5.Nf3, driving the queen away and gaining a tempo ; however, 3...Nf6 more common in modern practice.
There are also many other third moves, some of the more respectable are:
3.Nc3 – the Mason Gambit, or Keres Gambit
3.d4 – the Villemson Gambit or Steinitz Gambit
3.Be2 – the Lesser Bishop's Gambit or Tartakower Gambit
3.Qf3 – the Breyer Gambit or Hungarian Gambit.
Other moves have been assigned a name too but are very rare in serious tournaments.
[ edit] Classical Variation: (black responses)
The Classical Variation arises after 3.Nf3 g5,
when the main continuations traditionally have been 4.h4 (the Paris Attack)
, and 4.Bc4.
However, recently also 4.Nc3 (the Quaade Attack) has been played by strong players.
After 4.h4 g4 White can choose between 5.Ng5 or 5.Ne5. 5.Ng5 is the Allgeier Gambit intending 5...h6 6.Nxf7, but is considered dubious by modern theory.
Stronger is 5.Ne5, the Kieseritzky Gambit, which is relatively positional in nature. It was used very successfully by Wilhelm Steinitz, and was used by Boris Spassky to beat Bobby Fischer in a famous game at Mar del Plata 1960. This motivated Fischer into developing his own defense to the King's Gambit – see "Fischer Defense" below.
Instead of 4.h4, the extremely sharp Muzio Gambit arises after 4.Bc4 g4 5.0-0!? gxf3 6.Qxf3, where White has gambited a knight but has three pieces bearing down on f7 . Such wild play is rare in modern chess, but Black must exercise care in consolidating his position. Black can avoid the Muzio by meeting 4.Bc4 with 4...Bg7 and ...h6. 
Perhaps the sharpest continuation is the Double Muzio after 6...Qf6 7.e5 Qxe5 8.Bxf7+!? leaving white two pieces down in eight moves, but with a position some masters consider having equal chances.   (There is also a Triple Muzio with 8.b3!? instead of 8.Bxf7+!?, offering the rook on a1; if black captures the rook with 8...Qxa1 9.Nc3, the black queen is " stalemated " and White can sometimes force draws with his rook and queen.) 
[ edit] Becker Defence
The Becker Defence (3.Nf3 h6), has the idea of creating a pawn chain on h6, g5, f4 to defend the f4 pawn while avoiding the Kieseritzky Gambit; Black will not be forced to play ...g4 when White plays to undermine the chain with h4. White has the interesting option of 4. b3, though the main line continues with 4.d4 g5 and will usually transpose to lines of the Classical after 5.d4 d6 6.Bc4 Bg7.
[ edit] Bonch–Osmolovsky Defence
The rarely-seen Bonch–Osmolovsky Defence (3.Nf3 Ne7) was played by Mark Bluvshtein to defeat former world title finalist Nigel Short at Montreal 2007, though it has never been highly regarded by theory. 
[ edit] Cunningham Defence
The Cunningham Defence (3.Nf3 Be7) is Black's most aggressive option; it can permanently prevent White from castling after 4.Bc4 Bh4+ 5.Kf1 (else the wild Bertin Gambit, or Three Pawns' Gambit, 5.g3 fxg3 6.0-0 gxh2+ 7.Kh1, played in the nineteenth century). In modern practice, it is more common for Black to simply play 4...Nf6 5.e5 Ng4, known as the Modern Cunningham.
(3.Nf3 Nf6) – intending 4.e5 Nh5, holding onto the pawn – is considered somewhat inferior and is rarely played today. In one of the lines, White can usually obtain a crushing attack via a rook sacrifice, i.e., 4.e5 Nh5 5.d4 g5 6.h4 g4 7.Ng5 Ng3 8.Bc4! Nxh1 9.Bxf7+ Ke7 10.Nc3 (looking for immediate mate at d5, or later via queen at f6) and Black appears doomed .
[ edit] Modern Defence
The Modern Defence, or Abbazia Defense, (3.Nf3 d5) has much the same idea as the Falkbeer Counter-Gambit, and can in fact be reached by transposition, e.g. 2.f4 d5 3.exd5 exf4. Black concentrates on gaining piece play and fighting for the initiative rather than keeping the extra pawn. It has been recommended by several publications as an easy way to equalize, although White keeps a slight advantage due to his extra central pawn and piece activity. If White captures (4.exd5) then Black may play 4...Nf6 or recapture with 4...Qxd5, at which it point it becomes the Scandinavian Variation of KGA.
[ edit] Fischer Defence
"The refutation of any gambit begins with accepting it. In my opinion the King's Gambit is busted. It loses by force." — R. Fischer, A Bust to the King's Gambit The (3.Nf3 d6), although previously known, was advocated by Fischer Defense Bobby Fischer after he was defeated by Boris Spassky in a Kieseritzky Gambit at Mar del Plata 1960. Fischer then decided to refute the King's Gambit, and the next year the published Fischer's analysis of 3...d6, which he called "a high-class waiting move" American Chess Quarterly  
The point is that after 4.d4 g5 5.h4 g4 White cannot continue with 6.Ne5, as in the Kieseritzky Gambit, and 6.Ng5 is unsound because of 6...f6!. This leaves the move 6.Ng1 as the only option, when after six moves neither side has developed a piece. The main alternative to 4.d4 is 4.Bc4, but it is considered inferior.
[ edit] 3...Nc6
Joe Gallagher writes that 3.Nf3 Nc6 "has never really caught on, probably because it does nothing to address Black's immediate problems." Like Fischer's Defense, it is a waiting move. An obvious drawback is that the Nc6 may prove a target for the d-pawn later in the opening. 
King's Gambit Declined
Black can decline the offered pawn, or offer a countergambit.
Kings Gambit (C30)
[ edit] Panteldakis Countergambit
Among the oldest countergambits in KGD is the Panteldakis Countergambit, 1.e4 e5 2.f4 f5?!, known from a game played in 1625 in which Gioachino Greco used it to win with the Black pieces. It is nonetheless considered dubious because 3.exf5 with the threat of Qh5+ gives White a good game. 
[ edit] Falkbeer Countergambit
The Falkbeer Countergambit is named after 19th-century German speaking Austro-Hungarian Ernst Falkbeer. It runs 1.e4 e5 2.f4 d5 3.exd5 e4, in which Black sacrifices a pawn in return for quick and easy development. It was once considered good for Black and scored well, but White obtains some advantage with the response 4.d3!, and the line fell out of favour after the 1930s.
[ edit] Nimzowitsch Countergambit
A more modern interpretation of the Falkbeer is 2...d5 3 exd5 c6!?, as advocated by Aron Nimzowitsch. Black is not concerned about pawns and aims for early piece activity. White has a better pawn structure and prospects of a better endgame. The main line continues 4.Nc3 exf4 5.Nf3 Bd6 6.d4 Ne7 7.dxc6 Nbxc6, giving positions analogous to the Modern Variation of the gambit accepted.
[ edit] Classical Defence
A common way to decline the gambit is with 2...Bc5, the "classical" KGD. The bishop prevents White from castling and is such a nuisance that White often expends two tempi to eliminate it by means of Nc3–a4, to exchange on c5 or b6, whereupon he may castle without worry. It also contains an opening trap for novices: if White continues with 3.fxe5?? Black continues 3...Qh4+, in which either the rook is lost (4.g3 Qxe4+, forking the rook and king) or White is checkmated (4.Ke2 Qxe4#). This line often comes about by transposition from lines of the Vienna Game or Bishop's Opening, when White plays f2–f4 before Nf3.
[ edit] Other moves
Other options in the KGD are possible, though unusual, such as the sharp countergambit 2...Nc6
3.Nf3 f5, advocated by Tony Miles; 2...d6, when after 3.Nf3, best is 3...exf4 transposing to the (though 2...d6 invites White to play 3.d4 instead); and 2...Nf6 3.fxe5 Nxe4 4.Nf3 Ng5! 5.d4 Nxf3+ 6.Qxf3 Qh4+ 7.Qf2 Qxf2+ 8.Kxf2 with a small endgame advantage, as played in the 1968 game between Fischer Defense Bobby Fischer and Robert Wade in Vinkovci. The greedy 2...Qf6 (known as the  Norwalde Variation), intending 3...Qxf4, is considered dubious. Also dubious is the Keene Defense: 2... Qh4+ 3.g3 Qe7 and Mafia Defense: 1.e4 e5 2.f4 c5.