A simple derivation is given to arrive at an expression for the Coulom
b energy of a polar crystal with emphasis on its dependence on the ext
ernal form of the crystal. It is found that the existing treatments ar
e incomplete, since they refer to the central cell in the crystal, whi
ch may have an energy different from the average one. It is stressed t
hat there is a very simple relation between results from calculations
using a cutoff scheme and results from Ewald summation. Explicit formu
las for the form-dependent contribution are developed for the case of
a rectangular crystal. However, the usual approach in which that contr
ibution is neglected appears to be a reasonable one for practical appl
ications.