When two or more different monomers unite together to polymerize, the product is called a copolymer and the process is called copolymerization. Copolymers that are obtained by copolymerization of two monomer species are sometimes termed bipolymers, those obtained from three monomers terpolymers, those obtained from four monomers quaterpolymers, etc. Commercially relevant copolymers include acrylonitrile butadiene styrene (ABS), styrene/butadiene co-polymer (SBR), nitrile rubber, styrene-acrylonitrile, styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate, all formed by chain-growth polymerization. Commercial examples formed by step-growth polymerization are the nylon-12/6/66 copolymer of nylon 12, nylon 6 and nylon 66, as well as the copolyester family.