Tourna Quasi Gut 16 Tennis String Packages : Multifilament Synthetic. Quasi-Gut is constructed with thousands of microfibers. Each is impregnated and contained in a premium thermo-elastic polyurethane resin. These tightly wound microfibers deliver a natural gut-like feel and playability without the high cost of gut. The premium PU resin greatly increases durability and weather shielding as compared to natural gut.