The Yosemite Decimal System

Class 1

Hiking

Class 2

Simple scrambling, with possible occational use of the hands.

Class 3

Scrambling; a rope might be carried.

Class 4

Simple climbing, often with exposure. A rope is often used. A fall on Class 4 rock could be fatal. Typically, natural protection can be easily found.

Class 5

Where rock climbing begins in earnest. Climbing involves the use of a rope, belaying, and protection (natural or artificial) to protect the leader from a fall.