The city of Adelaide, South Australia, was planned by Colonel William Light who now lies buried beneath a monument in the city topped with a surveyor's theodolite. But why is the theodolite supported by a tripod? Why not a support with two legs? one leg? four or more legs? What is so special about the number 3? Suppose the instrument is standing on a plane. Algebraically, the plane has equation of the form ax + by + cz + d = 0. There are four coefficients here, but since cancelling out any common factor leaves the equation unaltered, the plane is determined by the three ratios a : b : c : d. This corresponds to the fact that a plane is determined by three general points, implying that the tripod is the best support.
