A simplified method of calculating the crystallographic features of a diffusionless (martensitic) phase change is described and applied to the gold‐cadmium alloy cubic‐orthorhombic transformation. Use of the method requires a knowledge of the initial and final crystal structures and the specification of the crystallographic plane and direction of the inhomogeneous shear. It is based on the requirement that after partial transformation the interface between the two phases be free from stress on a macroscopic scale. The method involves representing on a stereographic net (a) the loci of directions in the original phase unchanged in length by the structure changeper seand (b) the loci of directions unchanged in length by the inhomogeneous shear. The intersection of these loci determine possible undistorted interface planes. Excellent agreement with experimental observations is found when the method is applied to the cubic‐orthorhombic transformation in gold‐cadmium alloys.