Parallel: From geometry to the footnote mark

To most people, parallel means running side by side without converging: railway tracks, the lines on a ruled page, the edges of a book. In mathematics and geometry, parallel lines maintain constant distance from each other and, by definition, never intersect. The concept extends into everyday language to mean similar, equivalent, or happening at the same time: parallel universes, parallel parking, running parallel investigations.

In industrial printing, parallel has become something entirely different. It is a typographical mark consisting of two short vertical lines placed together, typically breaking the left margin of a page. A printer or typesetter uses the parallel symbol to flag a passage in the main text and direct the reader's eye to an annotation, usually a footnote, endnote, or marginal gloss. Where an asterisk or dagger might mark the first note, a parallel marks the second, third, or subsequent reference point. The symbol sits just before or after the word requiring explanation, then appears again at the foot of the page or in the margin, anchoring the reader to the corresponding note.

This shift from a geometric concept to a referential mark happened because printers needed a visible, unmistakable way to connect text to its apparatus. Unlike the words 'see note' or a raised number, the parallel symbol is compact, repeatable, and instantly recognizable on a crowded page. The double-bar structure may have been chosen precisely because its parallel nature recalled the idea of correspondence: two things running alongside each other, one visible and one hidden, both essential to meaning.

In mechanical engineering, the term takes yet another form. A parallelogram of forces is a diagram in which two vectors representing simultaneous forces are drawn as adjacent sides of a parallelogram, meeting at a common point. The diagonal of that parallelogram represents the resultant force. Here, parallel returns to its mathematical roots, but the symbol itself carries no footnote function. The industrial printing sense remains confined to the typographical domain, where it serves as shorthand for 'look elsewhere for elaboration'.