Aid climbing is a style of climbing in which standing on or pulling oneself up via devices attached to fixed or placed protection is used to make upward progress. The term contrasts with free climbing in which progress is made without using artificial aids: a free climber ascends by only holding onto and stepping on natural features of the rock, using rope and equipment merely to catch them in case of fall and provide belay. In general, aid techniques are reserved for pitches where free climbing is difficult to impossible, and extremely steep and long routes demanding great endurance and both physical and mental stamina. While aid climbing places less emphasis on athletic fitness and raw strength than free climbing, the physical demands of hard aid climbing should not be underestimated. In early versions of the Yosemite Decimal System, aid climbing was class 6, but today the YDS uses only classes 1-5. Aid climbing has its own ranking system, using a separate scale from A0 through A6. Technique In a typical ascent with aid the climber places pieces of equipment called protection in cracks or other natural features of the rock, then clips a ladder-like device, called an aider, stirrup or étrier, to the protection, stands up on the aider, and repeats the process. Just as in free climbing, the usual aid technique involves two climbers, the leader and belayer. The leader is connected by a rope to the belayer, who remains at the belay station while the leader moves up. As the leader advances, the rope is let out by the belayer, and clipped by the leader into the pieces of protection as they are placed. If the leader falls, the belayer locks off the rope and, assuming the protection doesn't pull out, catches the leader's fall on the rope. When the leader, moving up, reaches the end of the rope, or a convenient stopping point, they build an anchor, hang on it, and affix the rope to it. This then becomes the next belay station. The belayer then ascends the fixed rope using mechanical ascenders, retrieving the protection that was placed by the leader. Meanwhile, the leader sets up a hauling system and, using another rope brought up for that purpose, hauls up a bag containing the climbers' food, water, hammocks or porta-ledge, sleeping bags, and so on. Many variations on this basic technique are possible, including solo