Morphological and optical effects
At morphological level, twinning introduces re-entrant angles that, in principle, are specific marks.
Except for twins by merohedry, the orientation of the optical indicatrix changes through the interface between adjacent crystals (individuals). Thus, apart particular orientations, if observed by a polarizing microscope between crossed polars, a twin never can be brought by rotation to a position of complete extinctions (dark field).
Overlap of lattices
Twin lattice, twin index and twin obliquity are entries related to the overlap of lattices.