Feature, common to many single crystals, where the crystal lattice is differently orientated in two or more parts of the crystal, e.g. twin plane, twin axis, etc., and simple or multiple twins. See albite twin; carlsbad twin; complex twins; contact twin; deformation twinning; geniculate twin; growth twinning; interpenetrant twin; mimetic twins; normal twins; parallel twins; pericline twinning; swallowtail twinning; transformation twinning; twin axis; twin law; twin plane.