The Prusik hitch as first described by Dr. Karl Prusik in 1931 was the beginning of rope ascent as we know it today.
The prusik hitch is a friction hitch that will grip the climbing rope and not move when weighted, if tied and dressed correctly.
According to Smith and Padgett's "On Rope" (p.137): "The standard three-knot system is the standard of comparison for all other ascending rigs. By becoming proficient with the three-knot system, the rope technician gains an understanding of the basic functions of ascending upon which all other systems are based. It is still a popular and useful system, a testimony to its functional ability."
The three-knot system is nothing more than three loops of rope attached separately to the climbing rope, in ascending order, with prusik hitches. The two lower loops will accommodate the climber's feet, with one foot in each loop, the upper loop is attached to the climber's harness. The climber ascends by standing in the footloops while advancing the harness loop, then sitting in the harness while advancing the footloops.
The double-wrap prusik was the hitch first described and used. The triple-wrap configuration is now accepted as the standard, by some, as the one most acceptable for life-support applications. The triple-wrap, because of its increased grip is more difficult to advance up the rope, but it is this same grip that increases its safety.
It is necessary that the rope used for prusiks be of a smaller diameter than that of the main climbing rope. The smaller their diameter, and the softer and more flexible the rope used, the more "grip" will be experienced between the hitch and the climbing rope. Prusik slings can be nothing more than loops created by using a double-fisherman's knot to create an "endless" loop.