Theory commonly refers to the erudite study and the philosophy behind the moves in chess (like opening or endgame theory) but this tag can also be used for questions about the game theoretic study of the game itself.

Sometimes this applies to specific openings (e.g., the Marshall Attack in the Ruy Lopez) and sometimes it relates to the game overall, such as discussions about positional play or hypermodern openings. The common theme is the search for a deeper understanding of the game.