Editing Sphere packing (section)

== Classification and terminology ==
A '''[[lattice (group)|lattice]]''' arrangement (commonly called a '''regular''' arrangement) is one in which the centers of the spheres form a very symmetric pattern which needs only ''n'' vectors to be uniquely defined (in ''n''-[[dimension]]al [[Euclidean space]]). Lattice arrangements are periodic. Arrangements in which the spheres do not form a lattice (often referred to as '''irregular''') can still be periodic, but also '''[[aperiodic]]''' (properly speaking '''non-periodic''') or '''[[randomness|random]]'''. Because of their high degree of [[symmetry]], lattice packings are easier to classify than non-lattice ones. Periodic lattices always have well-defined densities.