Someone mentioned the Pitzen. I use this knot a lot for tippet to fly. I use a double Pitzen (well actually, a double 16-20) to tie mono leader to flouro tippet. It is tied similarly as a blood knot is to the clinch. Someone gave me the hint - since I am so proficient with this 16-20 knot it was easy to do the splice this way. I did learn the knot held much better when I pull the tag ends very tight before seating the knot.