THE ONLY REAL RULE in rock climbing is: Do it safely. Climbing is inherently risky, and safety must always be the paramount concern. A climber cannot take any short cuts in establishing his protection. Becoming overconfident or hurrying are prescriptions for disaster.

But beyond safety lie a series of standards about how one is supposed to climb. For instance, the object is to climb smoothly but not fast, using the rope only for protection. Some climbs -- such as the so-called big walls of Yosemite Valley in California -- can only be accomplished by using aids, such as slings to stand in. Many first ascents that were made using aid devices have later been climbed "free" -- that is, with the protection of a rope but without using slings to stand in or using a piton for a hand or foot hold.

The difficulty of climbs has gradually come to be rated on a fairly uniform basis across the country. Climbs that involve some scrambling but on which no one would feel the need for the protection of a rope are 3rd class climbs. Those with more difficult scrambles on which some people would like a rope are 4th class. Those on which most climbers would like protection, or on which it is absolutely necessary, are rated as 5th class.

The 5th class is further divided on a decimal scale that runs from 5.0, the easiest, to 5.13, the most difficult. A beginning climber can usually move up very quickly from a 5.0 or 5.1 to a 5.4 or 5.5 level. Then things become a bit more difficult. A climb with a 5.9 rating is very difficult indeed, and most climbers never achieve that level, much less go beyond it.

Routes are rated on the basis of the most difficult single for a single difficult climbing move. Or it could be solid 5.7 and 5.8 from bottom to top, a much more difficult and strenuous proposition if the route is at all long.

Climbs that require the use of aids are rated 6th class, A1 through A4, with A1 routes being well protected and A4 routes not well protected at all.

Ratings can usually be trusted as a guide to the relative difficulty of routes within a given climbing area, though individual climbers may well find that, say, a 5.8 move on a particular route might be easier for them than a different type of move rated 5.7.

Ratings may be harder to compare when different climbing areas are involved. For instance, one expert climber, Mike Kehoe of Great Falls, Va., who has climbed both places, says a 5.5 rating at Seneca Rocks probably would be a 5.7 in the Tetons of Wyoming, where altitude and difficult approaches must also be considered.