The perfection loop is a quick and clean loop knot that works great at the end of leaders and fly line butt sections to interconnect each other. The perfection loop is commonly used in pre-packaged leaders to connect the leader to the fly line. Perhaps the most popular fly fishing knot, the perfection knot is strong and effective, able to be made into a very small loop.
The perfection loop is a great choice for heavy monofilament or fluorocarbon lines. Just make sure to wet it with saliva or water before tightening to ensure the leader is lubricated.
Jake Ehlers walks you through tying the perfection loop in the video below using a rope, so you can easily see each step.