  Crystals are paradigms of ordered structures. While order was once seen as
synonymous with lattice periodic arrangements, the discoveries of
incommensurate crystals and quasicrystals led to a more general perception of
crystalline order, encompassing both periodic and aperiodic crystals. The
current definition of crystals rests on their essentially point-like
diffraction. Considering a number of recently investigated toy systems, with
particular emphasis on non-crystalline ordered structures, the limits of the
current definition are explored.
