Lefty Kreh is credited with popularizing this knot.  It is strong, and depending on which of the following variations used test at either 90% or 100% of line strength, is easy to tie, and makes a nice open loop.  This is my preferred loop for connecting leader to fly, as the open loop allows for more movement and action of the fly.  In addition to using this loop to tie on a fly, I often use it to create loops in my leader for quick-change loop-to-loop connections.

Step 1

Begin by making a simple overhand knot in the standing line, then pass the tag end of the line through the eye of the hook.  Pass the tag end back through the overhand knot.  Make sure it goes back through on the same side it came out of the overhand knot. 

Step 2

 

Illustration 2

This option produces 100% knot strength and a wider loop

 

 

Wrap the tag end around the standing line between 3 to 7 turns as follows.

 

For 50 or 60 pound line - 3 turns,

 

For 14 to 40 pound line - 4 turns,

 

For 8 to 15 pound line - 5 turns,

 

And for lines less than 8 pound test - 7 turns.

 

After making the turns around the standing line, insert the tag back through the loop in the overhand knot.  If you pass it through, entering the opposite side of the loop that it exited as in illustration 2, the finished loop knot will be a little wider  and will test at near 100%.  This variation is usually preferred for connecting tippet to fly..

 

 

Illustration 2a

This option produces a narrower loop, but test at 90 -95% line strength.