If you climb on a single 60m rope and you wish to abseil back down the route you have just climbed, you can only abseil in 30m pitches in order to recover your rope. If, however, you have been using double ropes of 60m, then by tying the two ends together you can abseil 60m at a time.
Unless you abandon gear you can only abseil off a tree or an in-situ belay. With a single rope you would thread one end through a ring, maillon or karabiner, depending on the belay, or take one rope around a tree. Then take the two ends together and feed them over the edge. If you suspect that the ends may not reach the ground, it is good practice to tie them together to stop you abseiling off the ends.
With double ropes, you thread one of your ropes, through the ring, maillon or karabiner, or round the tree, and put the other rope alongside with the two bottom ends together. Making sure your tails are long enough, tie an overhand knot. The side with the knot will be the rope to pull to retreive your ropes. You need to make sure the free ends of both ropes are on the top so that when you toss them down the crag there is less likelihood of them getting tangled. It is good etiquette to look down the crag, if possible, shout ‘below’ before you toss them.
The overhand knot is one of the easiest knots to tie, but its’ best feature is that it will roll over an edge rather than get snagged. Some climbers use a Double Fisherman’s knot but this has a tendency to get stuck on edges and it is difficult to untie after it has been loaded.
It is recommended that if it is very wet, icy, or the two ropes are of a different thickness a second overhand knot is tied to sit on the first knot. As with any knot, always dress it, i.e. ensure there are no twists.
Whether using a single or a double ropes to abseil, both sides of the belay device are used and clipped into a
screwgate karabiner that is extended and attached to the belay loop. You may wish to safeguard your descent using a Prusik. The French Prusik is used below the extended belay device and attached to the belay loop via a screwgate karabiner. This avoids the possibility of the Prusik releasing if you choose not to extend the belay device and clip the screwgate into the leg loops – if knocked unconscious the body can twist and the back up prusik may slip up to the belay plate causing yoy to plumet earthwords. and release it.