There are two types of twinning: merohedral or hemihedral non-merohedral or epitaxial For merohedral twinning the diffraction patterns from the different domains are completely superimposable. Hemihedral twinning is a special case of merohedral twinning. It only involves two distinct domains. Pseudo-merohedral twinning is a subclass merohedral twinning in which lattice is coincidentally superimposable. In the case of non-merohedral or epitaxial twinning the reciprocal lattices do not superimpose exactly. In this case the diffraction pattern consists of two (or more) interpenetrating lattices, which can in principle be separated.