King's Knight Opening

The King's Knight Opening arises after 1. e4 e5 2. Nf3. The knight goes to its most natural square, strikes at the e5-pawn, and prepares a later d2-d4. It also follows the classical habit of developing a knight before the bishops, whose best squares are less clear this early. Classified as ECO C40, it is one of the most frequently reached positions in chess, and the engine rates it roughly balanced. Black must decide straight away whether to defend e5 or to counterattack.

The main reply by a wide margin is 2...Nc6, chosen in close to two-thirds of games. It develops a piece, guards e5 and covers d4, and it leads to the most heavily analyzed open games: 3. Bb5, the Ruy Lopez or Spanish Game; 3. Bc4, the Italian Game; and 3. d4, the Scotch. A less common option is 3. g3, the Konstantinopolsky, which fianchettoes the king's bishop instead.

The second most popular reply is 2...d6, the Philidor Defense. It holds e5 with a pawn and was once favored by theorists who wanted to avoid the Spanish. The cost is a slower development. The f8-bishop is hemmed in, and d4 is left uncontested, so 3. d4 is White's most testing answer. The move remains common in amateur play but is much rarer in tournaments. The third main choice, 2...Nf6, is Petrov's Defense, also called the Russian Game. Black counterattacks e4 rather than defending e5, and after 3. Nxe5 Black has several ways to win the pawn back. The resulting symmetrical positions have made it a byword for solidity and a frequent drawing weapon at elite level.

The remaining replies are sidelines. 2...d5, the Elephant Gambit, and 2...f5, the Latvian Gambit, also hit e4 but are considered dubious with best play. Against the Elephant, both 3. exd5 and 3. Nxe5 are playable. Against the Latvian, both 3. exf5 and 3. Nxe5 are possible. On Lichess both gambits score unusually well for Black. The Busch-Gass Gambit, 2...Bc5, gives up e5 for play against f2, either with ...Qh4 or by 3. Nxe5 Nc6 4. Nxc6 dxc6 with quick development. The Gunderam Gambit, 2...c6, aims for ...d5 but allows 3. d4 exd4 4. Qxd4. Among the passive defenses, 2...Qf6 (the McConnell or Greco Defense) takes the f6-square from the knight, and 2...Qe7 (the Gunderam Defense) blocks the f8-bishop. 2...f6, the Damiano Defense, weakens the king so badly that a knight sacrifice on e5 followed by a queen check on h5 exposes it. It has been regarded as refuted since the 16th century, and it scores worst of all the common replies.