Theoretical analysis of the ferroelastic domain structure of a La0.95Sr0.05Ga0.9Mg0.1O2.925crystal in threedifferent crystallographic phases is presented. Parameters of these configurations are obtained using group theo-retical approach, the method of spontaneous deformation as well as theoretical interpretation of twinning resultingfrom mechanical deformation (mechanical twinning theory). In the three phases of La0.95Sr0.05Ga0.9Mg0.1O2.95—trigonal, orthorhombic and monoclinic — the parameters of ferroelastic domain structures are determined; namelythe quantity of orientation states, symmetry elements of connection between states, orientations and types ofdomain walls, tensors of spontaneous deformations of the perovskite-type cells for every orientation state, elementsof twin shifts, which are needed for the reorientation of some orientation states to others. By using the foundparameters of bidomain configurations a mechanism is proposed, which causes chevron-like domain configurationsin compounds with martensitic phase transitions.