The Yosemite Decimal System (YDS) was developed by the Sierra Club in the 1930's to describe hiking and climbing routes. The classes attempt to specify the level of difficulty and technicality for a "typical person."
Five classes were delineated as follows:
Class 1. Walking, with no use of hands.
This is the simplest form of mountain travel requiring little or no technical skill. Class one refers to both on trail and off trail travel. If you're walking but you're not using your hands, then it's class one, trail or no trail.
Class 2. Walking, but hands are used for balance.
This is still a walking class, but here the terrain is so rough that one must use one's hands to steady oneself and maintain balance.
Class 3. Lowest level of climbing. Hands are used for upward or downward motivation. Hand and foot holds are well placed, obvious, and solid. No technical equipment required.
This is often referred to as "scrambling." Here, you're actually using your hands to move about the mountain, however little technical skill is required. If holds are few and far between, it's not class three. If holds require much in the way of special technique, it's not class three. If holds are tiny (e.g. fingertips only), it's not class three.
Class 4. Middle level of climbing. Hands are used for upward or downward motivation. Hand and foot holds are scattered and small. Special techniques are required. Safety equipment is typically used but may not be depending on the skill and confidence of the climber, quality of the rock, degree of exposure, etc.
Class 5. Highest level of climbing. Hands are used for upward or downward motivation. Hand and foot holds
are scattered and small. Special techniques are required. Safety equipment is typically used.
Class 5 is typically subdivided into grades such as 5.0, 5.5, 5.13, etc. The larger the number after the decimal point, the more difficult the climb. Class 5 is an open ended scale. If a more difficult climb is completed than was previously known, a new level is created. Currently, the most difficult climbs are rated at 5.15. In some cases, there is an even further sub division, denoted by a
lower case letter such as 5.13a, 5.13b, 5.13c, etc, with "a" being the
easiest and those letters further into the alphabet being progressively more
The difference between Class 4 and Class 5.
is no clear and obvious distinction between class 4 and 5 that I've
ever seen articulated anywhere. Both are technical climbing classes.
They are simply separated by degree of difficulty.
Subjectivity in Ratings.
Every effort was made to be objective in the development of the YDS classes, but inasmuch as they were developed by humans and refer to acts executed by humans, some degree of subjectivity exists. A hand hold that might be obvious to one person may not to another. What might be comparatively easy to one person might not be to another. However, despite the clearly human origin of the ratings, there is a surprising degree of unanimity regarding the ratings on well known routes. The climbing community tends to function on consensus. The difficulty lies in lesser known routes. On such routes, there may be some disagreement as to the exact classification.
Regional Differences. Routes that are rated class three in California, particularly in the Sierra Nevada, are frequently harder than routes rated class three elsewhere. For example, many routes in the Sierra Nevada that are rated class three would be rated as a class four if they were in Colorado. Why? Well, many of the routes in the Sierra Nevada were pioneered by none other than climbing legend Norman Clyde. Norman Clyde would rate climbs as class three and for him they were. Other people would probably rate the climb as a class four, but no one wants to go on record as changing a rating established by such a great climber. There's some controversy as to the above, with many people acting outraged that there might be the slightest subjectivity to the YDS classes. The reality is that there is a degree of subjectivity and that ratings do vary by region.
If you read other descriptions of the YDS classes, you may see more discussion of the level of risk. My bias is to describe routes in terms of physical difficulty and technicality. Therefore, my discussion of the level of risk is quite limited. My feeling is that the level of risk must be evaluated separately. Trying to combine two elements into a single measure results in inaccuracy in both.