Origins
The word 'line' is one of the most versatile in the English language, with more distinct senses than almost any other common noun. The Oxford English Dictionary devotes one of its longest entries to it. Yet this semantic sprawl traces back to something remarkably humble: a thread of flax.
Old English 'līne' meant 'a rope, a cord, a string' — a physical object, not an abstraction. It was borrowed from Latin 'līnea,' which meant 'a linen string' and, by extension, 'a mark or stroke drawn with such a string.' Latin 'līnea' was derived from 'līnum,' meaning 'flax' — the plant from which linen fiber is spun. Latin 'līnum' descends from Proto-Indo-European *līno- (flax), a word with cognates across the language family: Greek 'línon' (λίνον, flax, linen, cord), Old Irish 'lín' (flax, net), Lithuanian 'lìnas' (flax), and Old Church Slavonic 'lĭnŭ' (flax). Flax was one of the earliest cultivated plants in human history, domesticated around 7000 BCE, so the word is likely very ancient.
The semantic journey from plant to geometry happened through technology. Ancient builders, surveyors, and architects created straight lines by the simplest method imaginable: stretching a linen cord taut between two points. The Romans called this cord a 'līnea.' When the cord was chalked and snapped against a surface, it left a straight mark — also a 'līnea.' The abstract geometric concept of a line — a one-dimensional figure extending infinitely in both directions — was thus born from the most concrete of practices.
Old English Period
English 'linen' is a close relative: it comes from Old English 'līnen' (made of flax), from the same Latin 'līnum.' 'Lingerie' — borrowed from French in the 19th century — ultimately derives from the same root through Old French 'linge' (linen undergarments). 'Linseed' is 'flax seed' (from Old English 'līn' + 'sǣd'). Even 'linoleum' belongs to this family: it was invented in the 1860s and named from Latin 'līnum' (flax, because linseed oil is a key ingredient) + 'oleum' (oil).
The expansion of 'line' through English has been extraordinary. The 'rope' sense survives in fishing line, clothesline, and lifeline. The 'mark' sense gives us the lines on a page, the lines of a drawing, and lines on a face. The 'row' sense produces a line of people (queue), a line of soldiers, a product line, and an assembly line. The 'boundary' sense appears in state lines, property lines, and the thin blue line. The 'connection' sense gives us telephone lines, railway lines, and airline routes. The 'ancestry' sense yields lineage and bloodline.
The word 'linear' (from Latin 'lineāris') entered English in the 17th century to describe anything relating to lines. 'Delineate' (from Latin 'dēlineāre,' to sketch out from lines) came in the 16th century. 'Align' (from French 'aligner,' to put in a line, from 'à ligne') arrived in the 15th century. All of these keep the original metaphor visible.
Word Formation
The phrase 'to read between the lines' — to detect hidden meaning — dates from the 1860s. 'To draw the line' — to set a limit — is from the 1790s, originally referring to a line drawn on the ground in sports or combat to mark a boundary. 'To toe the line' — to conform to rules — derives from the practice of runners placing their toes on a starting line. 'Line of duty,' 'front line,' 'bottom line,' 'party line,' 'line of work' — the compound phrases are nearly inexhaustible.
In mathematics, the formalization of the line concept was pivotal. Euclid defined a line as 'breadthless length' — an entity with one dimension and no width. This definition, stripped of all physical properties, represents the ultimate abstraction of what began as a linen cord. The development of analytic geometry by Descartes in the 17th century made lines representable as algebraic equations, and the concept became foundational to calculus, linear algebra, and modern physics.
Perhaps the most philosophically resonant use of 'line' is in the concept of lineage — one's line of descent. Here the metaphor is of a thread running through time, connecting generations. This sense appeared in English by the 14th century and preserves the original Latin metaphor: a 'līnea' was a genealogical thread as well as a physical cord. The expression 'end of the line' works in both the spatial and genealogical senses — the final point of a physical line, or the end of a family's descent.