International
Tables for Crystallography Volume D Physical properties of crystals Edited by A. Authier © International Union of Crystallography 2013 
International Tables for Crystallography (2013). Vol. D, ch. 3.4, p. 535
Section 3.4.4.5. Ferroelastic domain twins and walls. Ferroelastic twin laws^{a}Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ18221 Prague 8, Czech Republic, and ^{b}Department of Mathematics and Didactics of Mathematics, Technical University of Liberec, Hálkova 6, 461 17 Liberec 1, Czech Republic 
As explained in Section 3.4.3.6, from a domain pair of ferroelastic singledomain states with two perpendicular equally deformed planes p and one can form four different ferroelastic twins (see Fig. 3.4.3.8). Two mutually reversed twins and have the same twin symmetry and the same symmetry of the twin pair . The ferroelastic twin laws can be expressed by the layer group or, in a less complete way (without specification of reversibility), by the twin symmetry . The same holds for two mutually reversed twins and with a twin plane perpendicular to p.
Table 3.4.4.6 summarizes possible symmetries of ferroelastic domain twins and corresponding ferroelastic twin laws . Letters V and W signify straindependent and strainindependent (with a fixed orientation) domain walls, respectively. The classification of domain walls and twins is defined in Table 3.4.4.3. The last column contains twinning groups of ordered domain pairs from which these twins can be formed. The symbol of is followed by a symbol of the group given in square brackets. The twinning group specifies, up to two cases, a class of equivalent domain pairs [orbit ] (see Section 3.4.3.4). More details on particular cases (orientation of domain walls, disorientation angle, twin axis) can be found in synoptic Table 3.4.3.6. From this table follow two general conclusions:

This follows from simple reasoning: all twin symmetries in Table 3.4.4.6 have been derived in the parent clamping approximation and are expressed by the orthorhombic group or by some of its subgroups. As shown in Section 3.4.3.6.2, the maximal symmetry of a mechanically twinned crystal is also . An additional simple shear accompanying the lifting of the parent clamping approximation cannot, therefore, decrease the symmetry derived in the parent clamping approximation. In a similar way, one can prove the statement for the group of the twin pairs and .