Opening theory
Opening theory is the body of knowledge about the opening, made up of the known moves, plans and evaluations of the main lines that players have tested over many games.
Also called: theory
Opening theory is the collected knowledge about how the first moves are best played. It comes from centuries of practice and analysis, and today engines and databases extend it. It records which moves are considered best, which lines are risky and what plans each structure suits.
Theory is organized by opening and variation. A main line is the most tested path, and sidelines are less common choices. In some lines, theory runs more than twenty moves deep, and the two players may both know it to the end. In others, it stops after a few moves, and you are on your own.
Knowing theory helps in two ways. It saves time and avoids early mistakes, and it shows typical pawn structures and plans. The second use matters more for most players. A move you recognize is a good starting point, but understanding why it is played is what lets you keep going when theory ends.
Theory changes. A line that was strong years ago may be refuted by a new idea, and a forgotten line may come back. A common mistake is to treat theory as fixed, or to learn only moves. Study the games that illustrate the line, and check where the main games turn from theory to original play.
A neighboring idea is Book move. A book move is a move in a well established opening line, one that already appears in published analysis, opening databases or an engine's prepared opening list, so a player can choose it from knowledge rather than calculation.
Also worth reading is Opening explorer. An opening explorer is a tool that shows, for any position reached from the start, which moves players have chosen in past games and how often each move led to a win, draw or loss.
Also worth reading is Tabiya. A tabiya is a standard, well known position that arises in a particular opening after a sequence of typical moves, and from which the main plans and ideas of both sides are understood.
Opening ideas such as "Opening theory" are easier to keep when they are tested on your own games, which is what a first lesson with George Li begins with.
Related terms
The opening is the first stage of a chess game, running from the starting position until most pieces have left their home squares and the kings are safe, and it shapes what kind of middlegame both players will face.
Opening preparation is the work done before a game to know your chosen lines well, including studying your own repertoire and checking the likely choices of a specific opponent.
A novelty is a move that has not been played before in a known position of an opening, introduced to take the opponent out of their preparation and into unexplored territory.
The main line is the sequence of moves most often played in an opening, the one considered the most critical test, and the one that theory has analyzed in the greatest depth.
A book move is a move in a well established opening line, one that already appears in published analysis, opening databases or an engine's prepared opening list, so a player can choose it from knowledge rather than calculation.
A player is out of book when the game reaches a position that neither side is recalling from opening knowledge or preparation, so from that move on the choices have to be worked out at the board.
Guides that go further
Most popular chess openings and win rates
The most played openings, how often White wins, and how the numbers change with rating, computed from public games.
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
A game review, a read on your level and a written study plan from George Li.
Want to spot this in your own games?
A first lesson starts with your games: George reviews them and builds a study plan around what he finds.