Scientific Anglers Mastery XTS Gel Spun Fly Line Backing is thin and strong and exteremly abrasion resistant. This style of backing is popular for use on large arbor reels because of the ability to fit more backing onto the reel due to its thin diameter. This line is the preferred backing for saltwater fly fishing. The Gel Spun construction makes this line impervious to UV rays, salt and other chemicals that can break down other lines. Hi-vis yellow in color.