A Duncan Loop Knot (also called a “uni-knot” or a “Grinner”) is one of several fly fishing knots you'll want to know how to tie. Use it to make a sliding loop to attach your tippet to your leader. The knot can either be left open or tightened against a hook eye. The Duncan Loop can also be used to join two lines of similar diameter. The Duncan Loop works especially well for heavy monofilament.
Step 1--Select the Correct Tippet
Select the right size (0X being the strongest, down to 8X) by dividing the fly size by 4. For a size 8 fly, for example, you’d use a 2X tippet.
Step 2--Tie the Knot
Pass the tag end of the tippet through the hook eye. Make a loop around the standing end. Wrap the end around the standing end inside the loop five times.
Step 3--Finish It Off
Lubricate the knot before tightening it to strengthen it. Use water or saliva. Tighten the knot by pulling the tag end. Trim the tag ends of the leader and tippet with the nippers. Cut close enough to leave room for a bit of movement without the knot coming undone.