A compression ring with only the top side tapered. Like on the keystone ring, the tapered side (keystone angle 7) causes the axial clearance to vary as the ring moves radially and thus, reduces ring sticking. Owing to its asymmetrical cross section, the ring has a positive twist when installed.A half keystone ring is used in the top groove of passenger car and truck diesel engines when a rectangular ring is no longer adequate in regard to ring sticking but a keystone ring is not yet warranted. Another application is in 2-stroke gasoline engines, e.g. for snowmobiles and ultra light aircraft.