Аннотация:The objective of the present work is to develop a method of finding principal stress trajectories in pressure-independent and pressure-dependent plasticity under plane strain and plane stress conditions. It is shown that the geometry of the principal line trajectories is defined, in the general case, by a telegraph equation. In obtaining this general result, two special cases are lost: those corresponding to the cases where one and two families of characteristics are straight. In the case of Tresca and Coulomb-Mohr yield criteria, the results presented are independent of any flow rule that may be chosen to calculate the deformation and also independent of whether elastic strains are included.