A chess opening is the group of initial moves of a chess game. Recognized sequences of opening moves are referred to as openings as finished by White or defenses, as finished by Black, but opening is also used as the general term. There are many dozens of different openings, and hundreds of named variants. The Oxford Companion to Chess lists 1,327 named openings and variants. These vary widely in character from quiet positional play to wild tactical play. In addition to referring to specific move sequences, the opening is the first phase of a chess game, the other phases being the middlegame and the endgame.
A sequence of opening moves that is considered standard (often cataloged in a reference work such as the Encyclopedia of Chess Openings) is referred to as “the book moves”, or simply “book”. These reference works often present these move sequences in simple algebraic notation, opening trees, or theory tables. When a game begins to deviate from known opening theory, the players are said to be “out of book”. In some opening lines, the moves considered best for both sides have been worked out for twenty to twenty-five moves or more. Some analysis goes to thirty or thirty-five moves, as in the classical King’s Indian Defense and in the Sveshnikov and Najdorf variations of the Sicilian Defense. Professional chess players spend years studying openings, and continue doing so throughout their careers, as opening theory continues to evolve. Players at the club level also study openings but the importance of the opening phase is smaller there since games are rarely decided in the opening. The study of openings can become unbalanced if it is to the exclusion of tactical training and middlegame and endgame strategy.
A new sequence of moves in the opening is referred to as a “theoretical novelty“. When kept secret until used in a competitive game it is often known as a “prepared variation“, a powerful weapon in top-class competition.
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 Aims of the opening
 Top-level objectives
At higher levels of competition, for many years the main objectives of opening play were to obtain the better position when playing as White and to equalize when playing as Black. The idea behind this is that playing first gives White a slight initial advantage; for example, White will be the first to attack if the game opens symmetrically (Black mirrors White’s moves).
Since about the 1950s another objective has gradually become more dominant. According to IM Jeremy Silman, the purpose of the opening is to create dynamic imbalances between the two sides, which will determine the character of the middlegame and the strategic plans chosen by both sides. For example, in the main line of the Winawer Variation of the French Defense (1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5 c5 5.a3 Bxc3+ 6.bxc3), White will try to use his bishop pair and space advantage to mount an attack on Black’s kingside, while Black will seek simplifying exchanges (in particular, trading off one of White’s bishops to blunt this advantage) and counterattack against the weakened pawns on White’s queenside; both players accept different combinations of advantages and disadvantages. This idea was a doctrine of the Soviet school of chess.
A third objective, which is complementary to the previous ones and has been common since the 19th century, is to lure the opponent into positions with which the player is more familiar and comfortable than the opponent. This is usually done by transpositions, in which a game that apparently starts with one opening can reach a position that is normally produced by a different opening.
 Common aims in opening play
Irrespective of whether they are trying to gain the upper hand as White and equalize as Black or to create dynamic imbalances, players generally devote a lot of attention in the opening stages to:
- Development: One of the main aims of the opening is to mobilize the pieces on useful squares where they will have impact on the game. To this end, knights are usually developed to f3, c3, f6 and c6 (or sometimes e2, d2, e7 or d7), and both players’ King and Queen pawns are moved so the bishops can be developed (alternatively, the bishops may be fianchettoed with a manoeuvre such as g3 and Bg2). Rapid mobilization is the key. The queen, and to a lesser extent the rooks, are not usually played to a central position until later in the game, when many minor pieces and pawns are no longer present.
- Control of the center: At the start of the game, it is not clear on which part of the board the pieces will be needed. However, control of the central squares allows pieces to be moved to any part of the board relatively easily, and can also have a cramping effect on the opponent. The classical view is that central control is best effected by placing pawns there, ideally establishing pawns on d4 and e4 (or d5 and e5 for Black). However, the hypermodern school showed that it was not always necessary or even desirable to occupy the center in this way, and that too broad a pawn front could be attacked and destroyed, leaving its architect vulnerable; an impressive-looking pawn center is worth little unless it can be maintained. The hypermoderns instead advocated controlling the center from a distance with pieces, breaking down one’s opponent’s center, and only taking over the center oneself later in the game. This leads to openings such as Alekhine’s Defense – in a line like 1. e4 Nf6 2. e5 Nd5 3. d4 d6 4. c4 Nb6 5. f4 (the Four Pawns Attack), White has a formidable pawn center for the moment, but Black hopes to undermine it later in the game, leaving White’s position exposed.
- King safety: The king is somewhat exposed in the middle of the board. Measures must be taken to reduce his vulnerability. It is therefore common for both players either to castle in the opening (simultaneously developing one of the rooks) or to otherwise bring the king to the side of the board via artificial castling.
- Prevention of pawn weakness: Most openings strive to avoid the creation of pawn weaknesses such as isolated, doubled and backward pawns, pawn islands, etc. Some openings sacrifice endgame considerations for a quick attack on the opponent’s position. Some unbalanced openings for black, in particular, make use of this idea, such as the Dutch and the Sicilian. Other openings, such as the Alekhine and the Benoni, invite the opponent to overextend and form pawn weaknesses. Specific openings accept pawn weaknesses in exchange for compensation in the form of dynamic play. (See pawn structure.)
- Piece coordination: As each player mobilizes his or her pieces, each attempts to assure that they are working harmoniously towards the control of key squares.
- Create positions in which the player is more comfortable than the opponent: Transposition is one common way of doing this.
Apart from these ideas, other strategies used in the middlegame may also be carried out in the opening. These include preparing pawn breaks to create counterplay, creating weaknesses in the opponent’s pawn structure, seizing control of key squares, making favourable exchanges of minor pieces (e.g. gaining the bishop pair), or gaining a space advantage, whether in the centre or on the flanks.
 Opening repertoires
Most players realize after a while that they play certain types of positions better than others, and that the amount of theory they can learn is limited. Therefore, most players specialize in certain openings where they know the theory and which lead to positions which they favor. The set of openings a player has specialized in is called an opening repertoire. The main elements a player needs to consider in a repertoire are:
- As White, whether to open with 1.e4, 1.d4, 1.c4, or 1.Nf3
- As Black, a defense against any of these openings
A very narrow repertoire allows for deeper specialization but also makes a player less flexible to vary against different opponents. In addition, opponents may find it easier to prepare against a player with a narrow repertoire.
The main openings in a repertoire are usually reasonably sound, that is, they should lead to playable positions even against optimal counterplay. Unsound gambits are sometimes used as surprise weapons, but are unreliable for a stable repertoire. Repertoires often change as a player develops, and a player’s advancement may be stifled if the opening repertoire does not evolve. Some openings which are effective against amateur players are less effective at the master level. For example Black obtains active play in return for a pawn in the Benko Gambit; amateur players may have trouble defending against Black’s activity, while masters are more skilled at defending and making use of the extra pawn. Some openings which are played between grandmasters are so complex and theoretical that amateur players will have trouble understanding them. An example is the Perenyi Attack of the Sicilian Defense (see diagram) which yields an immensely complicated and tactical position that even strong players have difficulty handling, and that is beyond the comprehension of most amateurs.
 Opening nomenclature
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Major changes in the rules of chess in the late fifteenth century increased the speed of the game, consequently emphasizing the importance of opening study. Thus, early chess books, such as the 1497 text of Luis Ramirez de Lucena, present opening analysis, as does Pedro Damiano (1512), and Ruy López de Segura (1561). Ruy Lopez’s disagreement with Damiano regarding the merits of 2…Nc6 led to 3.Bb5 (after 1.e4 e5 2.Nf3 Nc6) being named for him as the Ruy Lopez or Spanish Opening. Opening theory was studied more scientifically from the 1840s on, and many opening variations were discovered and named in this period and later. Opening nomenclature developed haphazardly, and most names are historical accidents not based on systematic principles.
The oldest openings tend to be named for geographic places and people. Many openings are named after nationalities, for example Indian, English, Spanish, French, Dutch, Scotch, Russian, Italian, Scandinavian, and Sicilian. Cities are also used, such as Vienna, Berlin, and Wilkes-Barre. The Catalan System is named after the Catalonia region of Spain.
Chess players’ names are the most common sources of opening names. The name given to an opening is not always that of the first player to adopt it; often an opening is named for the player who was the first to popularize it or to publish analysis of it. Eponymic openings include the Ruy Lopez, Alekhine’s Defense, Morphy Defense, and the Réti Opening. Some opening names honor two people, such as the Caro-Kann.
A few opening names are descriptive, such as Giuoco Piano (Italian: quiet game). More prosaic descriptions include Two Knights and Four Knights. Descriptive names are less common than openings named for places and people.
Some openings have been given fanciful names, often names of animals. This practice became more common in the 20th century. By then, most of the more common and traditional sequences of opening moves had already been named, so these tend to be unusual or recently developed openings like the Orangutan, Hippopotamus, Elephant, and Hedgehog.
Many terms are used for the opening as well. In addition to Opening, common terms include Game, Defense, Gambit, and Variation; less common terms are System, Attack, Counterattack, Countergambit, Reversed, and Inverted. To make matters more confusing, these terms are used very inconsistently. Consider some of the openings named for nationalities: Scotch Game, English Opening, French Defense, and Russian Game — the Scotch Game and the English Opening are both White openings (White chooses to play), the French is indeed a defense but so is the Russian Game. Although these do not have precise definitions, here are some general observations about how they are used.
- Used only for some of the oldest openings, for example Scotch Game, Vienna Game, and Four Knights Game.
- Along with Variation, this is the most common term.
- Usually used to describe a line within a more general opening, for example the Exchange Variation of the Queen’s Gambit Declined.
- Always refers to an opening chosen by Black, such as Two Knights Defense or King’s Indian Defense, unless, of course, it has ‘reversed’ in front of it, which makes it an opening for white. The term “defense” does not imply passivity; many defenses are quite aggressive (such as the Nimzo-Indian Defense).
- An opening that involves the sacrifice of material, usually one or more pawns. Gambits can be played by White (e.g., King’s Gambit) or Black (e.g., Latvian Gambit). The full name often includes Accepted or Declined depending on whether the opponent took the offered material, as in the Queen’s Gambit Accepted and Queen’s Gambit Declined. In some cases, the sacrifice of material is only temporary. The Queen’s Gambit is not a true gambit because there is no good way for Black to keep the pawn (Ward 1999:10).
- A gambit played in response to another gambit, almost always by black. Examples of this include the Albin Countergambit to the Queen’s Gambit, the Falkbeer Countergambit to the King’s Gambit, and the Greco Counter Gambit (the former name of the Latvian Gambit).
- A method of development that can be used against many different setups by the opponent. Examples include London System, Colle System, Stonewall Attack, Réti System, Barcza System, and Hedgehog System.
- Sometimes used to describe an aggressive or provocative variation such as the Albin-Chatard Attack (or Chatard-Alekhine Attack), the Fried Liver Attack in the Two Knights Defense, and the Grob Attack. In other cases it refers to a defensive system by Black when adopted by White, as in King’s Indian Attack. In still other cases the name seems to be used ironically, as with the fairly inoffensive Durkin’s Attack (also called the Durkin Opening).
- Reversed, Inverted
- A Black opening played by White, or more rarely a White opening played by Black. Examples include Sicilian Reversed (from the English Opening), and the Inverted Hungarian. The Reti, King’s Indian Attack and Reversed Sicilian (from the English), and other “Black played by White with an extra tempo,” often start with 1. Nf3 or 1. c4.
A small minority of openings are prefixed with “Anti-“. These are openings intended to avoid a particular line otherwise available to one’s opponent, for example the Anti-Marshall (against the Marshall (Counter) Attack in the Ruy Lopez) and the Anti-Meran Gambit (against the Meran Variation of the Semi-Slav Defense).
 Classification of chess openings
For a list of openings as classified by the Encyclopedia of Chess Openings, see List of chess openings.
The beginning chess position offers White twenty possible first moves. Of these, 1.e4, 1.d4, 1.Nf3, and 1.c4 are by far the most popular as these moves do the most to promote rapid development and control of the center. A few other opening moves are considered reasonable but less consistent with opening principles than the four most popular moves. The Dunst Opening, 1.Nc3, develops a knight to a good square, but is somewhat inflexible because it blocks White’s c-pawn; also, after 1…d5 the knight is liable to be driven to an inferior square by …d4. (Note that after 1.Nf3 the analogous 1…e5? loses a pawn.) Bird’s Opening, 1.f4, addresses center control but not development and weakens the king position slightly. The Sokolsky Opening 1.b4 and the King’s and Queen’s fianchettos 1.b3 and 1.g3 aid development a bit, but they only address center control peripherally and are slower than the more popular openings. The eleven remaining possibilities are rarely played at the top levels of chess. Of these, the best are merely slow such as 1.c3, 1.d3, and 1.e3. Worse possibilities either ignore the center and development such as 1.a3, weaken White’s position (for instance, 1.f3 and 1.g4), or place the knights on poor squares (1.Na3 and 1.Nh3).
Black has twenty possible responses to White’s opening move. Many of these are mirror images of the most popular first moves for White, but with a tempo less. Defenses beginning with 1…c6 and 1…e6, often followed by the center thrust 2…d5, are also popular. Defenses with an early …d6 coupled with a kingside fianchetto are also commonly played.
The most important scheme of classifying chess openings for serious players is by ECO code, a series of 500 opening codes assigned by the Encyclopaedia of Chess Openings. Although these codes are invaluable for the serious study of the chess opening, they are not very practical for a broad survey of the chess opening as the codes obscure common structural features between related openings.
A simple descriptive categorization of the chess opening is King’s Pawn Openings, Queen’s Pawn Openings, and Others. Since these categories are still individually very large, it is common to divide each of them further. One reasonable way to group the openings is
- Double King Pawn, Symmetric or Open Games (1.e4 e5)
- Single King Pawn or Semi-Open Games (1.e4 other)
- Double Queen Pawn or Closed Games (1.d4 d5)
- Single Queen Pawn or Semi-Closed Games (1.d4 other)
- Flank openings (including 1.c4, 1.Nf3, 1.f4, and others)
- Unusual first moves for White