The Reuleaux triangle is a curious geometric shape: it looks like a rounded triangle, yet it has a remarkable property. Each side is an arc drawn from the opposite vertex, and together these arcs form a curve of constant width. In other words, no matter how you measure it—between two parallel lines—the width always remains the same.
Because of this property, a curve of constant width can act as a rotor inside a square. As it turns, the shape remains in contact with all four sides of the square at every moment, tracing a continuous motion while never leaving the boundary.
Mechanisms based on intermittent motion appeared early in mechanical engineering. One of their first practical uses was in sewing machines, where motion had to advance in precise steps rather than continuously. Today similar mechanisms are widely used in devices that move film frame by frame—such as cameras, projectors, and film-processing equipment—where controlled, stop-and-go motion is essential.
