Sir Roger Penrose is a professor of mathematics at Oxford University. One of his most fascinating discoveries was a series of aperiodic (or quasiperiodic) tilings. Remarkably, one of these consists of just two plane figures, both parallelograms. These extraordinary patterns are almost but not quite regular - small regions sometimes repeat, but there is no 'repeat' unit at any scale which could simply be copied to tile the plane (or your browser).
Find out more about Sir Roger Penrose.
Make your own Penrose tiling at The Gallery of Interactive Geometry.