Due to the spontaneous deformations of ferroelectric crystals in their ferroelectric phases, twins (domains) may be separated only by those composition planes (domain walls) which satisfy the mechanical compatibility conditions; such planes are in an infinite crystal called permissible walls. A mathematical treatment is proposed which makes it possible to find all permissible walls in any ferroelectric species. It is shown that different types of permissible walls exist: W∞ walls with arbitrary orientation, crystallographically prominent Wf walls, and S walls whose indices depend on PS and the electromechanical coefficients. The orientations of such S walls are necessarily temperature dependent. Two particular domains may be separated by a W∞ wall or by one of two mutually perpendicular permissible walls of Wf and/or S type. In a number of species, however, there are pairs of domains between which no wall is permissible. It is shown that the occurrence of W∞ and Wf walls can be predicted from the symmetry properties of the paraelectric phase. The properties of S walls are discussed in detail. Among the known ferroelectrics, S walls are most likely to exist in MASD and in boracites. The available data on nickel‐chlorine boracite suggest that S walls have been observed in this material.