# Family structure

By superposing two or more identical copies of the same polytype translated by a superposition vector (*i*.*e*. a vector corresponding to a submultiple of a translation period) a fictitious structure is obtained, which is termed a *superposition structure*. Among the ** family structure**: it exists only if the shifts between adjacent layers are rational, i.e. if they

The family structure is

common to all polytypes of the same family. From a group-theoretical viewpoint, building the family structure corresponds to transforming (“completing”) all the local symmetry operations of a space groupoid into the global symmetry operations of a space-group.*See also*

*Chapter 9.2 of International Tables of Crystallography, Volume C*