Symbols for OD groupoid families referring to OD structures (polytypes) consisting of more than one kind of layer

Principles for the construction of such symbols are given and exemplified for any of the four categories. The symbols characterize the symmetry relations common to all OD structures or polytypes of a family. They consist, as a rule, of two lines - the first containing the layer group symbols of any of the constituting OD layers, the second indicating the positional and/or symmetry relations between adjacent OD layers. Symbols in use, referring to OD crystals containing equivalent OD layers, result from an application of these principles. The principles are discussed for schematic examples and applied to kaolinite-type minerals and yttrium hydroxychloride. Further examples: Ca[B(OH)₄]₂.2H₂O, mica, chlorites, vermiculites and pyroxenes are quoted.