Opening theory

Opening theory is the accumulated knowledge about how positions in the opening, the first phase of a chess game, have been played and analyzed. It includes known moves, reasonable alternatives, typical plans, tactical dangers, and judgments about the resulting positions. It is not a rulebook or a list of compulsory moves. It summarizes what earlier games and analysis have taught.

A line is a specific sequence of moves. An evaluation is a judgment about who stands better and why, perhaps because of king safety, piece activity, or pawn structure. A book move is a move recognized within this published knowledge. Knowing that a move is theoretical is not enough; a player should also understand what problem it solves and what kind of position it aims to reach.

Public knowledge and personal preparation

Theory is a public body of material found in books, game databases, courses, and opening explorers. Home preparation is the personal selection a player studies for future games. A chess engine, meaning a program that calculates and evaluates positions, can test variations and uncover improvements, but its numbers still need to be interpreted in terms of plans and practical decisions.

• Which pieces should be developed and which squares need control.
• Which immediate tactics punish an inaccurate move.
• What kind of middlegame, the phase after the opening, usually follows.