The tecnifibre black code 4s tennis string reel is 110m 360ft long and is the official string of the atp world tour it is a polyester monofilament tennis string with a gauge of 17 125 and it has a square section that ensures an optimal level of spin the string maintains its tension well and boasts thermocare technology that delivers a great amount of flexibility the string is black and made of high density polyester and it provides 20 more power than the pro red code and 15 more durab