The heavy weight, .024 inch diameter, 49 strand, Nylon coated stainless steel. Strong, durable, will not rust, kink resistant, big - best for big beads, heavy beads and beads with big holes. Bigger stringing material fills the hole better so that the bead is less likely to sit on an angle. Use with large lampwork, big ethnic beads, or for anchoring your boat. (We're kidding on that last one. Sorta). 40 lb test.