In the design of a stressed skin metal airplane it is essential that correct stiffener stresses and effective widths of skin acting with the stiffener be known in order to insure tha t fictitious margins of safety in wing and fuselage analyses are not shown. In this paper the problem of determining these stresses and effective widths is discussed and some experimental results from Lockheed tests are presented. An experimental method is described which makes possible the accurate determination of these quantities, employing extensometer strain data interpreted by means of a compression stress-strain curve for the stiffener.