Schematic representations of (a) the MDO1 and (b) MDO2 polytypes of the hemi-MEK and (c) the fully ordered structure of the mono-benzene and mono-toluene solvates according to OD interpretation. The symmetry of the (I) layers is represented by large right-angled triangles which are black on one and white on the other side. Solvent molecules, which are located between (I) layers are represented by smaller obtuse-angled triangles. Shifts of the layers along the viewing direction are not indicated. Elements of layer groups and operations relating adjacent layers are indicated using the graphic symbols standardized in the International Tables for Crystallography, Vol. A (Hahn, 1983). For 22 and nr,s operations the standard symbols for 21 and n were used. Elements of total operations are painted red. In (a) and (b) the maximum equivalence regions (MERs) are indicated by brackets between layers and layer names.