Main line
A main line is a central, well-established continuation used to organize an opening's theory.
The label does not mean every opponent will choose it. It marks a branch that receives substantial theoretical attention and often shapes the opening's typical plans.
A main line is a central route through the theory
The Ruy Lopez has more than one heavily studied route. The Morphy and Berlin defenses both lead into central bodies of theory, so main line is an organizing label rather than one compulsory sequence.
Starting position · Position 0/10
Main line does not mean mandatory
A main line is not the only sound continuation, and it may not be the easiest branch for a new player to maintain. A quieter variation can be a better repertoire choice when its plans are clearer and its common positions are easier to recognize.
Main lines can change over time
When a branch is refuted, rehabilitated, or replaced by a more practical choice, books and databases may promote a different continuation. Treat the label as an editorial map of theory rather than a permanent rule.