Tensile strength is the stress at which a material breaks or permanently deforms. There are three definitions of tensile strength, yield strength, ultimate strength and breaking strength. Test pieces are screwed into or gripped in jaws and stretched by moving the grips apart at a constant rate while measuring the load and the grip separation. This data is plotted as load vs extension and then converted to engineering stress (load/original area) vs engineering strain (fractional change in length over the test section assuming the deformation is uniform). In special circumstances, the actual stress and strain may be calculated if the true cross section is measured during the test.