Main line
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.
Main lines are the paths that strong players choose most often and that have been tested in practice. Because so many games follow them, they are well mapped: the key positions, the standard plans and the typical mistakes are known.
Studying the main line is useful because it shows what both sides are trying to do in the opening. Even if you later prefer a sideline, knowing the central plans tells you what you are deviating from, and what the opponent is aiming for.
Main lines are not always the best choice for every player. They tend to hold more theory, so a player who follows one must remember more and may meet an opponent who has prepared deeply. A line that is popular today may also change as new ideas appear.
A common mistake is to memorize a long main line without understanding why the moves are played. Learn the pawn structure, the placement of the pieces and the typical breaks first, then fill in the exact moves. That way you can find a good move when the opponent changes the order, and you are not lost when the game leaves your memory.
Compare it with 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 Spanish setup. The Spanish setup is the White formation from the Ruy Lopez, with the bishop on b5 or a4 pressuring the knight on c6, and later c3, d4 and h3 to build a strong center and keep tension on e5.
It sits close to 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 "Main line" 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
A sideline is a less common or less critical branch of an opening that is considered playable but is not the move most often chosen at the highest level of 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.
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 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.
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.