To assess the aerodynamic merits of a particular configuration it is convenient if similarity parameters can be found that allow comparison with other vehicles at different Mach numbers and lift coefficients. A pair of such similarity parameters occurs naturally in the study of ‘direct’ and ‘interference’ lift. The use of these parameters assists in the optimization of lift per unit drag at supersonic speeds. The designer can, for example, use these improvements to reduce wing size and structure mass. This paper derives these similarity parameters and illustrates their use.