Rectangular and general quadrilateral plate elements having 12 degrees-of-freedom are treated in this paper. New elements are derived, two based upon assumed displacement fields and others upon the assumed-stress hybrid principle. Some of these elements account for the effects of transverse shear deformation. The hybrid elements admit the possibility of continuously variable element stiffness. The new elements, as well as closely related elements previously derived, are subjected to numerical tests in order to evaluate their relative merits. These tests include computation of displacements and moments in homogeneous and sandwich plates.