The simple lattices studied in chapter 3 cannot describe all the periodic structures that crystalline substances exhibit in nature. Indeed, more realistic models for the majority of crystals are given by the periodic arrangements of points called multilattices, which are finite unions of translates of a given simple lattice. Alternatively, multilattices can be thought of as one skeletal lattice (or skeleton) at whose points are placed congruent clusters of atoms: this gives the motif of the multilattice, which contributes in an essential way to characterizing the symmetry of the whole structure.