This is probably due to a degree of roughness in the surface of fibers, often imperceptible to the eye, yet preventing them when in close contact from slipping easily upon each other. This is greatly increased by twisting the fibers together, and is added to by the toughness of the fibers themselves, the whole giving to rope a great resisting power. In the case of wire rope it is the firmness with which the metal holds together that gives it its great resisting strength. It is also not unlikely that the pressure of gravitation takes part in rope making, by holding the fibers in close contact, even if we do not know how this force operates.