The Yosemite Decimal System is used to rate the difficulty of any terrain that can be traveled -- not just rock climbing routes.

Class 1 is walking, such as on a trail.
Class 2 is difficult cross-country travel. An example would be crossing a talus field.
Class 3 is where climbing begins. While hands might be used in Class 2 for balance, they are used in Class 3 terrain to actually hang on to the rock.
Class 4 is on steep rock with smaller holds and greater exposure. A fall is probably fatal.
Class 5 is difficult and steep rock climbing. It has been further subdivided by difficulty. Originally, the YDS ran from 5.0 to 5.9, with 5.9 considered to be the greatest possible difficulty that could be climbed unaided. Advances in rope, rock shoes and protection have blown the top off the scale. Today, the hardest known climb is 5.14d, though the scale has no theoretical maximum. Only ratings over 5.9 are assigned an a-d suffix.
Class 6 was once a designation used for aid climbing. Today, this type of rock climbing is instead rated from A0 to A5.

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