A prussic knot is one that it tied with a loop of rope around a rope that is thicker than itself. typically is this means accessory cord (tied into a loop either using a double or single overhand knot or a fisherman's bend) commonly carried by climbers on their harnesses as part of a "rack".
It is tied around a climbing rope and allows the climber to ascend the rope, either by using another prussic loop, or by using a single prussic loop and a mechanical ascender, sometimes a belay devices is used in place of the mechanical ascender, if a belay device is used it will an "auto locking*" device.
- auto locking belay devices may in some circumstances fail, for this reason it is important to have a fail safe, so that in the event the device fails, the climber does not hit the floor.
It is often used in the event where one climber is not able to complete a section of a climb, so the other climber proceeds to the top, and lowers the rope down so the second climber can ascend the rope and the two can carry on.