The double overhand is used occasionally as a stopper knot, tied in the end of the rope to prevent you from abseiling off the end.

It is more often used to tie loose ends off around the main strand of rope, or back up knots such as the rethreaded figure-of-eight or figure-of-eight on a bight.

It is also used as the basis for the double fishermans bend to tie two ropes together.

double_overhand.jpg (20965 bytes)
Pass the end of the rope over itself... 125-2538_img.jpg (23115 bytes)
... around the main part of the rope once more... 125-2539_img.jpg (22120 bytes)
... and then under itself and through the first loop formed. 125-2541_img.jpg (21158 bytes)
Pull the knot tight and set. 125-2542_img.jpg (21947 bytes)