Waxed linen is a strong, consistent material that works great for knotting. The wax coating on these cords makes well defined, crisp knots and holds the knots well in place. It comes in a wide variety of colors and thicknesses referred to as "ply". I use a 4-ply in most of my jewelry and that seems to be the standard medium size. A single strand of 4-ply will fit most beads. I tend to use multiple strands of it in my designs if I'm using larger beads. Plus, more strands gives me more design options when I'm beading and I also like the texture and fullness of the multiple cords.
Irish waxed linen cord is the best you can buy. You can find Irish waxed linen online at Bello Modo, Ornamentea and Royalwood, Ltd. Also, please visit our contributor, Tracy Statler's blog for more information about using Irish waxed linen cord. She also has some great jewelry ideas and tutorials! Check her out!