A co-poly string is made from polyester and a host of other additives. In this way, co-poly strings have emerged as a reaction to poly strings: They’re designed to avoid some of the problems offered by pure polyester strings.

“Co-poly” is generally referred to as this same string with other additives. Hunter Hines with Dunlop explains that additives are included in the mix to “bring about different physical properties (softer, more elastic, better tension maintenance, etc.)” than are found in the base poly.

Steve Crandall of Ashaway explains it this way: “Co-poly is a marketing term for poly strings with some additives in the polymer formation to modify the polyester’s negative properties.” Crandall also adds one tidbit that should not be overlooked. “No matter what additives are used,” he says, “the resulting string is much stiffer and less elastic than any other type of tennis string.”