Protects mono loops from wear if there is contact with a metal component. Useful for lighter mono that is to small for a thimble. What size do I need? Monofilament lines to 1.5mm diameter (Momoi 150lbs) size E Monofilament lines to 1.7mm diameter (Momoi 250lbs) size F Monofilament lines to 2.0mm diameter (Momoi 300lbs) size G Monofilament