The one-sided overhand bend is often referred to as the European Death Knot (EDK), due to the fact that it looks insufficient for tying two ropes together. Nevertheless, it is one of the best knots to use to tie two ropes of equal diameter together for an abseil. While the double fisherman's bend is a stronger knot, the EDK is easier to tie, easier to undo, and less prone to catching on sharp edges on the pull-down. However, this knot may be unsuitable for tying together older ropes, as the weaker knot combined with the loss of strength in older rope may be unsafe.

See the report from David Drohan and the Bushwalkers Wilderness Rescue Association on preferred knots for canyoning for more information.

Note that EDK sometimes refers to the figure-of-eight version of this knot. Do not use this to tie two ropes together. If set badly, it can unroll with less than 50kg of weight, and is extremely dangerous.

This knot should be tied with at least 30cm (12 inches) of tails in the free ends, to avoid any chance of it unrolling and the knot failing. It should only be used to tie together two ropes of the same or similar diameter.

See the link above, and also Rope Sport Myths for other links to information on this and other knots for tying two ropes together.

One-sided overhand bend
Hold the two ends together and pass them over the main ropes. EDK - slide 1
Feed the two ends through the loop formed... EDK - slide 2
... and pull tight and set the knot EDK - slide 3