Because the two-fold surface is periodic in one direction and aperiodic (with a quasiperiodicity related to the Fibonacci sequence) in the perpendicular direction, frictional anisotropy is not unexpected; however, the magnitude of that anisotropy in the Park experiment, a factor of eight, is unprecedented. In a 2005 article in Science, Park et al! Through various models, we investigate generic geometric mechanisms that might give rise to this anisotropy. measured in vacuum the friction between a coated atomic-force- microscope tip and the clean two-fold surface of an AlNiCo quasicrystal. By eliminating chemistry as a variable, the experiment also demonstrated that the low friction of quasicrystals must be tied in some way to their quasiperiodicity.