Double Fisherman's Knot

The Double Fisherman's Knot securely ties two ropes together or can be used to tie the ends of rope or cord together to form loops.

Another use for this knot is to make another knot more secure by tying this knot in the running end of the rope behind another knot, a practice common to mountain climbers.

In that case, you are effectively tying the Double Fisherman's around the standing line of the other knot.