https://www.ucda.com/objet-dart-french-curve/
Objet d'art: French Curve
Photo by Malerapaso
A French curve is a template usually made from metal, wood or plastic composed of many different segments of the Euler spiral (aka the clothoid curve). It is used in manual drafting, graphic design, and in fashion design to draw smooth curves of varying radii. 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. They were invented by the German mathematician Ludwig Burmester and are also known as Burmester (curve) set. Today, Adobe Illustrator has made the french curve all but obsolete.
Source: Wikipedia