If you have this knot completely wired, then congratulations. If you don't, then this blogpost is just for you...!
The double-fisherman's knot is a knot that may be used to join two ropes together. The ropes may be of similar or dissimilar diameters. It is a very secure knot. Indeed, it is so secure, that it is often recommended for cords that will be permanently tied together such as prussik loops.
The biggest problem with the double-fisherman's is that it is very difficult to untie once it has been loaded. As a result, it is not recommended for quick situations where you want to tie two ropes together, such as in rappels.
The Canadian Guide, Mike Barter has put together the following video on how to tie a double-fisherman's knot:
--Jason D. Martin