Subgroups of SO(3) Associated with Tilings
  We give a thorough analysis of those subgroups of SO(3) generated by rotations about perpendicular axes by 2\pi/p and 2\pi/q. A corollary is that such a group is the free product of the cyclic groups of rotations about the separate axes if and only if p,q\ge 3 and are both odd. These groups are naturally associated with a family of hierarchical tilings of Euclidean 3-space. 
Charles Radin and Lorenzo Sadun