[en] A crystallographic analysis of the ferroelectric/ferroelastic domains and phase boundaries in thin platelets of Pb(Mg_1/3Nb_2/3)O₃, associated with the electric-field-induced phase transition from the disordered cubic phase to the induced 3m phase, is reported. Calculations have been performed by taking into account the internal stresses and their relation to the unit cell parameters. The results have permitted us to interpret the experimentally observed polar macrodomain structure, phase boundary and switching of the induced polarization. A diagram connecting the possible domain types and the shapes of the m3m from or to 3m phase boundaries has been proposed. The role of the several domain types and features of their elastic matching have also been analysed. (author)