A compression ring with a wedge cross section. With its tapered sides, radial movement of the ring in engine operation will cause the axial clearance in the groove to increase and decrease. This greatly reduces ring sticking, as the ring continuously works its way free of the combustion residues. These rings are designed with an overall side angle of 6° bzw. 15o, the larger angle being more effective against the tendency to coking. The keystone ring is used in the top groove in passenger car and truck diesel engines where ring sticking must be expected.