X-ray diffraction patterns give Fourier transforms of the spatial arrangement of atoms in a material. For an ordinary crystal with atoms on a repetitive lattice, the patterns consist of a few isolated peaks. For quasicrystals with generalized Penrose tiling structures the patterns also contain a few large peaks, though as in example (b) on page 586 there are also a hierarchy of smaller peaks present. In general, materials with nested structures do not necessarily yield discrete diffraction patterns. In the early 1990s, experiments were done in which layers a few atoms thick of two different materials were deposited in a Thue-Morse sequence. The resulting object was found to yield X-ray diffraction patterns just like example (a) on page 586.