Main Line

Main line in chess is an ordinary English compound applied metaphorically, on the same railway model that gives main line its transport sense of a principal route from which branches diverge; Etymonline records the related railway sense of a side line as a line extending from a main line from 1890. Wiktionary gives no separate etymology for the chess sense and defines it as a standard sequence of opening moves considered to be best play, with its earliest chess citation shown being a 2012 New York Times column by Dylan Loeb McClain. The chess glossary defines a main line as the principal, most important, or most often played variation of an opening, contrasted with the branches that leave it. The underlying noun line is defined in chess as a sequence of moves, usually in the opening or in analysing a position. No source located in this session documents a first chess use or a coinage date for the term.

First documented: n.a. — n.a. (earliest chess citation shown at Wiktionary is a 2012 New York Times column by Dylan Loeb McClain, which is a usage example rather than a first attestation)