We review long-range Coulomb interactions in systems consisting of periodically repeated replicas of identical unit cells containing a net dipole moment. Cases of both infinite and large finite samples of arbitrary shape are considered. The general result due to Smith (1981 Proc. Roy. Soc. Lond. A 375 475) for a large finite sample is considerably simplified. This allowed us to reveal further its physical meaning. We show that extra shape-dependent terms both in the electrostatic potential and energy have a pure macroscopic origin. We also suggest an alternative derivation for the Coulomb potential of a large finite sample of arbitrary shape which is believed to be simpler than the original one due to Smith.