The origins of an unusual diffraction phenomenon which occurs in 1,3-dibromo-2,5-diethyl-4,6dimethylbenzene are discussed. A distinct 'hole' is observed in the disorder diffuse scattering distribution at wavevectors close to q = 0. If the diffuse intensity distribution is described in terms of concentration waves which modulate an average P2~/c structure, two different types of mode are possible, a 2;~ mode which tends to preserve a 21-screw axis relationship between neighbouring molecules and a -~2 mode which tends to preserve a c-glide plane relationship. We show that the observed diffuse scattering is practically completely explained in terms of modulations which are of the 2~2 symmetry, even though the determined average crystal structure (space group P21) corresponds to a frozen-in 21 mode. We conclude that the short-range order, which tends locally to preserve the c-glide plane, is due to dipole-dipole interactions between molecules, while the stability of the lattice over a long range (corresponding to wavevectors close to q = 0) results from the ability of the molecules to form a closely packed structure in three different crystallographic planes, and this is governed by the short-range repulsive interactions. The 'hole' represents a range of wavevectors for which the strain from the packing forces, resulting from the local breaking of the P21 symmetry, outweighs the energy gained from the short-range ordering of the molecular dipoles.