A tiling is called aperiodic if its hull contains only non-periodic tilings. The hull of a tiling formula_1 contains all translates "T+x" of "T", together with all tilings that can be approximated by translates of "T". Formally this is the closure of the set formula_2 in the local topology. In the local topology (resp. the corresponding metric) two tilings are formula_3-close if they agree in a ball of radius formula_4 around the origin (possibly after shifting one of the tilings by an amount less than formula_3).