Backup rings are designed to keep O-Rings from extruding. These Back-up Rings are produced in either a contoured design, OR split or solid, depending on your material choice. You will find stock materials such as Fluorocarbon (FKM, Viton®), Nitrile (BUNA), PEEK™, PTFE (Teflon®)
Square O-Rings also belong to the AS568 classification system. They are basically a standard O-Ring, but instead of round, they have a square profile.
They are interchangeable with Quad Rings, in most cases.