Guymont's (Phys. Rev. B (1981), 24, 2647) nondisruption condition is m
ade the basis for dividing phase transitions in crystals into nondisru
ptive and disruptive categories. Ferroic phase transitions are defined
as nondisruptive phase transitions involving a change of point-group
symmetry. Prototype symmetry is also defined rigorously in terms of th
e nondisruption condition. This approach forms the basis of a comprehe
nsive and informative tensor-classification scheme for twinning in cry
stals (full details of which are published elsewhere), as also of a cr
ystallographic classification of phase transitions.