Made with aramid fiber ( Kevlar ) the all new F-20, it is precisely 3-ply twisted and 0.016" diameter. Bonded with a special resin, rounding the thread as much as possible for improved grip. We have expanded the range of colors to offer another option for archers who do not have sophisticated tools to make or repair their strings. F-20 serving have a high grip power that makes them very suitable for recurve bow strings F-20 serving material is supplied in reusable reels made of sturdy polymer, containing approximately 90 meters.