The width of a figure in a given direction is the smallest width of all the strips perpendicular to that direction that contain the figure. For instance, the width of a unit square varies from 1 to as the direction changes from 0° to 45°.

Among the many interesting geometric features of a Reuleaux triangle is that (like a circle) it is a curve of constant width, as illustrated here. As the triangle rolls along the axis, its width remains constant. However, the center of the figure, whose path is shown in red, does not remain level as the triangle rolls.