A Reuleaux polygon is a polygon that is a curve of constant width - that is, a curve in which all diameters are the same length. The best-known version is the Reuleaux triangle. Both are named after Franz Reuleaux, a 19th-century German engineer who did pioneering work on ways that machines translate one type of motion into another, although it was known before his time.

A Reuleaux triangle.

The Reuleaux triangle is the simplest nontrivial example of a curve of constant width - that is, a curve in which all diameters are the same length. To construct the Reuleaux triangle, start with an equilateral triangle. Center a compass at one vertex and sweep out the (minor) arc between the other two vertexes. Do the same with the compass centered at each of the other vertexes. Delete the original triangle. The result is a curve of constant width. Equivalently, given an equilateral triangle T of side length s, take the boundary of the intersection of the disks with radius s centered at the vertexes of T.

By the Blaschke-Lebesgue theorem , the Reuleaux triangle has the least area of any curve of given constant width.

The Reuleaux triangle can be generalized to regular polygons with 2n + 1 sides. See Reuleaux polygon; also British coin Twenty Pence, British coin Fifty Pence.

Trivia

• Because all of its diameters are the same length, the Reuleaux triangle - actually, all Reuleaux polygons - is the non-obvious answer to the Mensa-like question "What shape can you make a manhole cover so that it cannot fall down through the hole?" The obvious answer is a circle.

• The rotor of the Wankel engine is a Reuleaux triangle.

• Although a Reuleaux triangle rolls smoothly and easily, it does not make a good wheel because it does not have a fixed center of rotation. While an object on top of rollers with cross-sections that were Reuleaux triangles (like using logs as rollers, but shaped like Reuleaux triangles) would roll smoothly and flatly, an axle attached to wheels shaped like Reuleaux triangles would bounce up and down three times per revolution.

• The existence of Reuleaux polygons is a good demonstration of why you cannot use diameter measurements to verify that an object has a circular cross-section.

Three-dimensional version

The intersection of the balls of radius s centered at the vertexes of a regular tetrahedron with side length s is called the Reuleaux tetrahedron , but is not a surface of constant width . It can, however, be made into a surface of constant width in two ways.