The Sierra Club rating system was first introduced in California's Sierra Nevada in 1936. The original system was modified in 1938 and is not only what we still use today but it is used in most regions throughout North America.
Class 1: Walking
Class 2: Difficult cross country travel often involving large and unstable talus. Occasional hands may be used for balance or a pull or two.
Class 3: Specific hand holds and foot holds become necessary but the holds are usually large and easy to locate. The exposure (distance above the ground) could be such that a climber feeling uncomfortable should be belayed with the climbing rope.
Class 4: Steep rock with smaller holds and a lot of exposure. Ropes and anchored belays become continuously used.
Class 5: Increasingly difficult rock climbing involving the use of protection placed between the lead climber and the belayer. Class 5 spans a wide range of difficulty. Therefore, it began to be broken down further with the addition of a decimal system, the YDS (Yosemite Decimal System). Originally intended to be 5.0 to 5.9. As the sport of rock climbing involved it opened the scale to 5.10, 5.11 and today 5.15 exists.
Climbing Grade
The grade of a route is designated by roman numerals I-VI. This grade was introduced in Yosemite Valley to represent the average time and overall committment involved in a route. It is sometimes misleading for high country routes as some count the time from leaving camp and some from when the class 5 climbing begins.
Grade I: A few hours.
Grade II: Up to half a day.
Grade III: A full day.
Grade IV: An extra long day often beginning before dawn and ending after dark. At times a bivouac may be necessary near the summit or on the descent.
Grade V: Usually involving one planned bivouac.
Grade VI: Usually involving multiple planned bivouacs.