Theme T stability is considered of flat rectangular plates when in-plane biaxial loads are applied. The in-plane and bending elastic properties are orthotropic and the plate axes of symmetry and the elastic axes of symmetry coincide. Both the initial buckling and the reduction in stiffness occurring at the instant of buckling are considered. Numerous authors have considered the stability of orthotropic plates under biaxial load. Wittrick, * for example, has shown for the simply supported case among others that the parameters involved can be combined into one curve giving the buckling load for any plate. Reference 2 considers the buckling and postbuckling of a class of biaxially loaded laminated plates in the presence of bending-extensional elastic couplings. The present paper treats the stability of specially orthotropic plates with these couplings absent; a further rationalization is then possible, in which for any plate the proportional loss of stiffness at buckling can also be expressed in a single curve. For a particular plate, the combination of these universal curves, for buckling load and the loss of stiffness at buckling, allows unified consideration of the stability of simply supported biaxially loaded orthotropic plates. It follows that the relative stiffness at buckling (i.e., the ratio of plate stiffness immediately after buckling to that immediately before) depends on one parameter involving the elastic constants and the transverse load; this fact may be exploited in design.