By effect of a twinning operation, both the direct and reciprocal lattice of the individuals forming a twin are overlapped. Overlapping (restoration) of nodes belonging to different individuals can be: (i) exact and total (twinning by merohedry); (ii) exact but partial (i.e. only a fraction of the nodes of an individual lattice is restored; twinning by reticular merohedry); (iii) total but approximate (twinning by pseudomerohedry), approximate and partial (twinning by reticular pseudomerohedry).
- twin index
The reciprocal n of the fraction 1/n of (quasi)restored nodes is called twin index
- twin lattice
The lattice that is formed by the (quasi)restored nodes is the twin lattice. It corresponds to the crystal lattice in twins by (pseudo)merohedry and to a sublattice of the crystal (individual) in twins by reticular (pseudo)merohedry.
- twin obliquity
The twin obliquity is a measure of the distortion of a (sub)lattice in twins by (reticular) pseudomerohedry.