Double Fisherman's Knot

The double fisherman's knot is great for making loops.  These loops are placed on working or climbing ropes and frequently used as a foothold.  To connect the loop formed by the double fisherman's knot, use a Prusik hitch. Once loaded, this knot is rarely untied.

It's also called a grapevine knot.

A video link to the one handed version is provided.  It also has the basic steps of making this knot.  Unique to the method on video is how an anchor hitch is formed first.  Then it's rolled into a Fisherman's knot. http://www.youtube.com/watch?v=kZ28hDW04ZE

Here's an example of a working system, where the double fisherman is used to form a loop.  In this case it will be a foot loop.

If you need some assistance with making a Prusik, this link can help: http://jimknowsknots.blogspot.com/2011/10/quick-prusik.html