A back-up ring is a ring of hard, extrusion resistant material such as high durometer nitrile, nylon, Hytrel, or PTFE. They are not seals themselves but are usually used in conjunction with O-Rings in high-pressure applications for support or as spacers. 

A back-up ring is designed to fit between the O-Ring and the extrusion gap and prevent the extrusion of the O-Ring. Back-up rings are installed with O-Rings and X-rings.  They prevent O-Rings from gap extrusion in static radial sealing. The hard back-up ring will keep the O-Ring material from flowing into the gap which will prevent premature failure. 

NBR backup rings have one side concave to fit the O-Ring. PTFE comes in single turn scarf cut (with an angle cut), solid or a spiral of two turns.