Scientific Anglers XTS GEL SPUN Fly Line Backing, 50 lb Test, YELLOW - 100, 150, 200, 250, 300, 400, 500 up to 3000 yds Micro-diameter gel spun polyethylene allows up to 75% more capacity than standard material Retains 100% strength while wet, extremely high abrasion resistance, high visibility, impervious to UV rays, gas, salt and other elements Smooth coating and low coefficient of friction (COF) Various lengths available wound from bulk spools of Scientific Anglers Gel Spun Backing. You only have to buy what you need - enough to fill one reel or buy in bulk.