A color–shape rating system is used to indicate the difficulty of trails (otherwise known as slopes or pistes).
There is no governing body that assigns difficulty ratings to ski trails. Instead, ski resorts assign ratings to their own trails, marking a given trail according to its relative difficulty when compared with other trails at that resort. As a result, identically pitched trails at different resorts can have different ratings. Some skiers and snowboarders may interpret this as manipulation of ratings of their slopes to appeal to as wide an audience as possible; in fact, it is an attempt by ski areas to conform to the trail rating conventions.
This means that a black diamond at one resort is not necessarily a black diamond at another; it could be a blue square or double black. Thus, the NSAA advises all skiers, of all ability levels, to work their way up from an easy slope so that they can become familiar with the trail markings at a ski area.
Although slope gradient is the primary consideration in assigning a trail rating, other factors come into play — including trail width, normal snow conditions and whether or not the resort regularly grooms the trail.
|Trail Rating||Symbol||Level of difficulty||Description|
|Green circle||Green Circle||Easiest||The easiest slopes at a mountain. Green Circle trails are generally wide and groomed, typically with slope gradients ranging from 6% to 25%<ref name="appG"> Template:Cite web</ref> (a 100% slope is a 45 degree angle).|
|Blue square||Blue Square||Intermediate||Intermediate difficulty slopes with grades commonly ranging from 25% to 40%.<ref name="appG"/> These slopes are usually groomed. Blue Square runs make up the bulk of pistes at most ski areas, and are usually among the most heavily trafficked.|
|Black diamond||Black Diamond||Advanced||Amongst the most difficult at a given mountain. Black Diamond trails tend to be steep (typically 40% and up)<ref name="appG"/> and may or may not be groomed, though the introduction of winch-cats has made the grooming of steep slopes both possible and more frequent.|
|Double black diamond||Double Black Diamond||Expert Only|| These trails are even more difficult than Black Diamond, due to exceptionally steep slopes and other hazards such as narrow trails, exposure to wind, and the presence of obstacles such as steep drop-offs or trees. They are intended only for the most experienced skiers.
This trail rating is fairly new; by the 1980s, technological improvements in trail construction and maintenance, coupled with intense marketing competition, led to the creation of a Double Black Diamond rating.
|Variations||Blue Square/Black Diamond||Various|| Variations such as doubling a symbol to indicate increased difficulty, or combining two different symbols to indicate intermediate difficulty are occasionally used. One example is a diamond overlapping a square to indicate a trail rating between a Blue Square and a Black Diamond. Mont Tremblant in Canada utilizes two blue squares right next to each other to indicate the same thing. Many resorts throughout Colorado use a double diamond with an "EX" in the center to mark a run with extreme terrain, even more difficult than a double diamond. Other U.S. resorts, such as Smugglers' Notch, Vermont, and Mt. Bohemia, Michigan, use triple black diamonds. The combination of symbols is comparatively rare at U.S. ski areas; most ski resorts stick to the standard 4-symbol progression (with the exception of the common EX runs in Colorado).
Non-standard symbols for standard ratings may be encountered at some ski areas. Bogus Basin, a resort near Boise, Idaho, uses orange diamonds on trailheads for trails that are indicated in the resort's Alpine Guide as double black diamonds.<ref name=bogus_basin_alpine_guide>Bogus Basin Mountain Recreation Area. 2010. Alpine Guide</ref>
|Terrain parks||Terrain Park||Various|| Terrain parks are whole or portions of trails that can offer a variety of jumps, half-pipes, and other special "extreme" sporting obstacles beyond traditional moguls. The trails are typically represented by an orange rectangle with rounded corners.
Usually, the terrain park will carry its own trail rating, indicating the level of challenge. A terrain park with a Black Diamond or Double Black Diamond rating would contain greater and more challenging obstacles than a park with a Blue Square rating.