This is an exceptionally strong, simple-to-tie knot for joining lines of different diameters. It's the best knot around for tying tippets to fly-fishing leaders, too.
Lay the line and leader alongside each other, overlapping six to eight inches.
Treating the two like a single line, tie an overhand knot, pulling the entire leader through the loop.
Leaving the loop of the overhand knot open, pull the tag ends of both the line and leader through again.
Hold both lines and both ends to pull the knot tight. Clip ends close to avoid foul-ups in the rod guides.