The tecnifibre black code 4s tennis string is the official string of the atp world tour and this reel is 200m 660ft long this monofilament string is made of high density polyester and has a square section made to provide a maximum level of spin thermocare technology keeps it highly flexible and tension maintenance is good the string is black and gives users 20 more power than the pro red code string and 15 more durability than the black code