  Domain boundaries and other twin boundaries in crystalline materials are
receiving increasing interest. They can carry unique functional properties,
which in many cases are absent in the surrounding bulk material. One such
property of domain boundaries can be their electric polarity. Phenomenological
insight in the polarity of domain boundaries was so far based either on the
knowledge of the order parameter and the form of Landau-Ginzburg free energy
functional, or on the knowledge of the symmetry of the domain boundaries. In
the present work we show on the concrete examples of potassium thiocyanate
(KSCN) and lacunar spinel crystals, that the concept of the primary
order-parameter can help to find the layer group describing the maximal
possible symmetry of a given domain boundary. Combination of layer group and
order parameter symmetries is then employed to clarify the nature of the
polarity of domain boundaries.
