ESDU Struct 02.01.24 provides data that allow the stiffness ratio to be determined of a plate under uniform compressive load applied parallel to its length. The edges of the plate are assumed to be simply-supported so that they are free to rotate but constrained to remain in the plane of the plate. The stiffness ratio is the ratio of the slope of the load-deflection curve after buckling to that before buckling, that is, the slope of the curve plotting the average stress in the plate against the stress in the long edges. A graph is given plotting the stiffness ratio against the ratio of the buckling half-wavelength to the plate width. It relates to all values of elastic lateral restraint from clamped to free applied to the long edges. To use it requires a knowledge of the buckling wavelength and for short plates (with an aspect ratio less than 3) ESDU 72019 provides values while for long plates values are tabulated in ESDU Struct 02.01.24 for various values of the lateral elastic restraint. The data apply to stresses lying in the elastic range.