A Golomb ruler is a set of marks such that all the differences between pairs are different. The differences from {0, 1, 4, 6} are all the numbers between 1 and 6; it is therefore called a perfect Golomb ruler and it is the largest such set.

If a circular wheel is used, a perfect set of measuring marks is called a difference set, a Ganymede circle, or an n-switch. All the distances between selected red points are different. The distances are indicated by arcs inside the circle.