Ruler

A ruler is a straightedge marked with units of length, used for measuring and for drawing straight lines. The most common rulers are flat strips made of wood, plastic, or metal, with lengths typically around 15 centimeters (6 inches). The edge along which measurements are read serves as the reference line and can be used to transfer marks to a workpiece.

Rulers carry scales in metric units (millimetres and centimetres) and imperial units (inches and fractions). Many rulers are dual‑marked, showing both systems. Precision depends on the edge quality and wear; accuracy can be affected by bending, warping, or marks that have faded with use.

Different types serve different tasks. School rulers are flat, transparent or opaque, and usually 15 or 30 centimetres long. Carpenter and steel rulers are rigid and may include imperial, metric, or both scales; folding rulers combine segments; flexible tape measures cover longer distances; architect rulers display multiple scales for technical drawings, while tailor’s rulers aid pattern work.

Rulers are used in education, carpentry, engineering, drafting, sewing, and laboratory work. They function in tandem with other instruments such as compasses, protractors, and calipers to ensure straight lines, straight edges, and accurately transferred measurements.

Historically, rulers emerged from early length standards and standardization efforts that culminated in metric and imperial systems. Modern rulers are produced to national and international tolerances and are produced in materials designed to maintain straightness, edge sharpness, and legibility of markings over time.