Aid climbing is any kind of climbing that uses aids, or tools, to help the climber. Aids can include ropes, hooks, hammers and other climbing tools. Aid climbing grades are used for routes with sections that are pretty much impossible to pass without the use of climbing aids. For example, most sheer, or vertical, rock walls would be impossible to climb safely without the use of ropes and hooks.
There are two main ratings systems used for aid climbing: the A system and the C system. The A system is used for general aid climbing, while the C system is used to rate what is known as a clean climb, or a climb that's completed without the use of a hammer. The reason for this difference is that some climbing locations don't allow the use of a hammer, which can cause damage to the rocks.
The A system is a rating from A0 to A6. As the number in the rating gets higher, the more difficult the climb is. For example, it may be more difficult to place a hook, or a hook may have a greater risk of coming loose, in higher grades. The A system also takes into account the danger of any potential fall, which depends on the height from which you're falling as well as what's at the bottom. A route with a higher grade in this system might present more danger if the climber falls.
The C system for rating clean aid climbs is exactly the same as the A system, except that it uses the letter C instead of A. It's only used when a hammer isn't included as one of the climbing aids. The use of a hammer can make a big difference in climbing, so a route might have a rating of an A4 with a hammer, but a C5 without one.
The Yosemite Decimal System (YDS) is also sometimes used to grade aid climbs. Part of the set of descriptions in the YDS allows for the use of climbing aids.
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