somewhat controversial, and updated versions of these classifications have been proposed. Class 1Hiking on a trail. Class 2Simple scrambling, with the possibility of occasional use of the hands. Little potential danger is encountered. Class 3 Scrambling with increased exposure. Handholds are necessary. A rope could be carried. Falls could easily be fatal. Class 4 Simple climbing, possibly with exposure. A rope is often used. Falls may well be fatal. Class 5 Is considered technical roped free climbing; belaying is used for safety. Un-roped falls can result in severe injury or death. Class 5 has a range of sub-classes, ranging from 5.0 to 5.15d, to define progressively more difficult free moves. Classification of climbs can vary quite a bit depending on location and history. Indoor gym ratings tend to be much higher ("softer," inflated) compared to outdoor ratings. Ratings tend to be lower ("old school" or "sandbagged") in outdoor areas where climbing routes were developed earlier. Guidebooks have often raised the ratings on older climbs to make them more comparable to newer ratings. For example, although The Trough at Tahquitz was originally the standard for 5.0, the 2001 edition of the standard guidebook rates it as 5.4. Grades The YDS grade system involves an optional Roman numeral grade that indicates the length and seriousness of the route. The grades are: The grade is more relevant to mountaineering and big wall climbing, and often not stated when talking about short rock climbs. Protection rating An optional protection rating indicates the spacing and quality of the protection available for a well-equipped and skilled leader. The letter codes chosen were, at the time, identical to the American system for rating the content of movies, except that there is no commonly recognized distinction between PG and PG13: The G and PG ratings are often left out as they are typical of normal, everyday climbing. R and X climbs are usually noted as a caution to the unwary leader. Application of protection ratings varies widely from area to area and from guidebook to guidebook. History Originally, hikers and climbers in the Sierra Nevada would describe routes relative to other routes. For example, Z was harder than X but easier than Y. This primitive system was difficult to learn for those who did not yet have experience of X or Y. In the 1930s, the Sierra Club therefore adapted a numerical system of classification that was easy