The Tecnifibre Black Code 4S tennis string reel is 110m (360ft) long & is the official string of the ATP World Tour. It is a polyester monofilament tennis string with a gauge of 17 (1.25) & it has a square section that ensures an optimal level of spin. The string maintains its tension well & boasts Thermocare technology that delivers a great amount of flexibility. The string is black & made from high-density polyester, & it provides 20% more power than the Pro Red Code & 15% more durability than the Black Code.