Aunty is right, it's the clinch knot. I personally use the palomar, uni.,or one that is easier than all the others called the Duncan loop. OOps, just exposed myself as a closet flyfisherman. Mark Sosin & Lefty Kreh wrote an excellent knot book years ago that has stood the test of times as well as the scrutiny of fisherman worldwide. I had the priveledge of having lunch with Lefty once and he suggested the Duncan loop. I have been using it ever since. It's not 100% but damn close, and rated just above the improved clinch. If you want the knots that are actually stronger than the rest look to the Bimini twist and Australian braid knots.The name of Lefty's book is Practical Fishing KnotsII.