Scientific Anglers XTS Gel Spun Fly Line Backing - If you're looking for the best backing available, nothing can beat the XTS gel spun backing. With a diameter a fraction of the standard material, XTS retains its strength in the water, offers extreme abrasion resistance, and can fend off the worst the elements have to offer. Micro-diameter gel-spun polyethylene allows up to 75% more capacity than standard material Retains 100% strength while wet High abrasion resistance, high visibility, impervious to UV rays, gas, salt, and other elements Smooth coating and low coefficient of friction (COF)