In theoretical plasticity, all work so far done has made use of slip lines as a frame of reference. In the present paper, stress trajectories are used instead, and results of theoretical interest and practical applicability are obtained. The equiareal property of stress trajectories as derived in the present paper can be readily used to evaluate photographs of slip lines in plasticity with a higher degree of accuracy and reliability than hitherto. The analytical method can be applied to the equiareal pattern formed by stress trajectories and formulas for stresses are obtained. An example referring to radial flow adequately illustrates the general equiareal property.