French curve

A French curve is a template usually made from metal, wood or plastic composed of many different curves. It is used in manual drafting to draw smooth curves of varying radii. The shapes are segments of the Euler spiral or clothoid curve. The curve is placed on the drawing material, and a pencil, knife or other implement is traced around its curves to produce the desired result.

As modern computer-aided design (CAD) systems use vector-based graphics to achieve a precise radius, mechanical templates (and most mechanical drawing techniques) have become obsolete. Digital computers can also be used to generate a set of coordinates that accurately describe an arbitrary curve, and the points can be connected with line segments to approximate the curve with a high degree of accuracy. Some computer-graphics systems make use of Bézier curves, which allow a curve to be bent in real time on a display screen to follow a set of coordinates, much in the way a French curve would be placed on a set of three or four points on paper.

French curves

A complete Burmester set from the Lexikon der gesamten Technik (1904)

This set of the three most common French curves is also known as a Burmester set. The one on the far left side is most commonly used for hyperbolas; the smaller one on the far right side is suited for ellipses. The large one below is used most for parabolas.

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