A crystallog. anal. of the ferroelec./ferroelastic domains and phase boundaries in thin platelets of Pb(Mg1/3Nb2/3)O3, assocd. with the elec.-field-induced phase transition from the disordered cubic phase to the induced 3m phase, is reported. Calcns. were performed by taking into account the internal stresses and their relation to the unit cell parameters. The results have permitted the authors to interpret the exptl. obsd. polar macrodomain structure, phase boundary and switching of the induced polarization. A diagram connecting the possible domain types and the shapes of the m.hivin.3m .dblarw. 3m phase boundaries is proposed. The role of the several domain types and features of their elastic matching also were analyzed.