The Surgeon's Knot is the fly angler's tippet-to-leader joiner. Lay the two ends parallel for 4 inches, then tie an overhand knot using both lines together — pulling the long ends through twice (some tie three times for slick fluorocarbon). Wet it, pull all four strands at once.
Faster than a Blood Knot but bulkier; better than a Blood Knot in the cold or when tying small. The standard for adding fresh tippet during a fishing day.