A load applied to a plate with an opening produces in certain conditions a local stress concentration near selected points on the opening edges. Near such points, known as stress concentrators, plasticity regions may develop. For a theoretical definition of plastic strains, a highly complex elastoplastic problem has to be studied. The difficulty is compounded by the fact that the shape of the plasticity region is not known in advance. An approximate distribution of plastic zones at the corners of a curvilinear square opening in a plate subjected to symmetric uniaxial extensions is formulated with respect to the plate's thickness.