The triangle is an ancient symbol of the godhead and the Trinity in Christianity. The rounded triangle is a triangle formed by the intersection of three arcs. Known geometrically as the Reuleaux triangle, it is the simplest curve of constant width other than the circle, which means it can rotate between two parallel lines touching its opposite edges. The geometry of rounded triangles was studied by the Swiss mathematician Leonhard Euler at the end of the eighteenth century and applied extensively to machines by the German engineer Franz Reuleaux in the nineteenth century. Its geometry is used in rotors, for instance, such as in pumps and the Wenkel engine. Rounded triangles are symbols of constant energy.