The Singletrack STS difficulty scale includes degrees of difficulty from the lowest S0 to the highest S5.

The STS scale evaluates exclusively the technical difficulties of a trail, whether flat, downhill or uphill, and is recognized internationally.

S0 Trail Difficulty Rating

S0 indicates a single trail that does not present particular difficulties.
Most of them are forest or meadow trails with good grip, or compact gravel. In this type of trails you will not face steps, rocks or roots. The gradient of the trail goes from mild to moderate and the curves are always quite wide. S0 trails can be tackled without special mountain bike technical skills.

In short
TRAIL CONDITIONS: compact terrain with good grip
OBSTACLES: none
GRADIENT: mild to moderate
CURVES: wide
MTB TECH SKILLS: no special skills required

S1 Trail Difficulty Rating

On an S1 trail you will find small obstacles such as not very prominent roots and small stones.
Often the increased difficulty rating is due to the drains or erosion. In S1 single trails the surface could be not very compact. The gradient reaches a maximum of 40% and no switchbacks will be encountered. Starting from S1 level, basic mtb riding techniques and good concentration are required. The most trickier passages require the ability to dose the brakes and to affect the trajectory by body displacement. Obstacles can all be rolled over with basic riding skills.

In short
TRAIL CONDITIONS: loose surface possible, with small roots and stones
OBSTACLES: small obstacles (drains, damage caused by erosion)
GRADIENT: <40%
CURVES: narrow
MTB TECH SKILLS: basic driving skills required

S2 Trail Difficulty Rating

Within the S2 classification you will find trails with larger roots, stones, steps and easy staircases.
Often there are narrow curves and gradient can reach 70% in some passages or sections. A fair amount of mtb riding ability is required to ridden obstacles.
The ability to brake at all time and to shift your center of gravity are necessary techniques, as well as the ability to precisely control the braking and to keep the body always active while driving.

In short
TRAIL CONDITIONS: surface most often loose, protruding roots and stones
OBSTACLES: obstacles of various kinds and staircases
GRADIENT: <70%
CURVES: easy switchbacks
MTB TECH SKILLS: advanced driving skills required

S3 Trail Difficulty Rating

Blocked single trails with many larger boulders and / or root passages belong to category S3.
There are often high steps, switchbacks and tricky traverses. You will have to deal also with slippery and not very compact terrain. Sections with a gradient over 70% are not rare.
S3 sections do not yet require trial technique, however excellent control of the bike, mastery in the brake modulation and an excellent balance are required.

In short
TRAIL CONDITIONS: technical, frequent protruding roots and large rocks, slippery ground and not very compact surface
OBSTACLES: high boulders
GRADIENT: >70%
CURVES: tight switchbacks
MTB TECH SKILLS: more than advanced driving skills required

S4 Trail Difficulty Rating

S4 describes very steep and blocked single trails with large block of rocks and / or challenging root passages, as well as loose surface.
Extremely steep slopes, narrow switchbacks and steps high enough to get in contact with the crown of the crank are common. To properly ride a S4 trail, trial techniques such as the ability to shift the front wheel and rear wheel, a perfect braking technique and an excellent balance are absolutely necessary. Only extreme bikers can tackle an S4 route. Even carrying a bike up such passages is often quite difficult.

In short
TRAIL CONDITIONS: technical, frequent protruding roots and large rocks, slippery and not compact terrain
OBSTACLES: steep ramps, very high steps (often at crown height or more)
GRADIENT: >70%
CURVES: very tight switchbacks
MTB TECH SKILLS: perfect mastery of the bike and trial skills such as displacement of the rear wheel in sharp turns.

S5 Trail Difficulty Rating

The S5 grade is characterized by a very technical terrain with counterslopes and slippery surface, extremely tight curves, extreme steepness (cliffs), very big obstacles in close sequence including fallen trees. Braking distance is very short if any. Some obstacles can only be jumped over. Hairpins are so tight that shifting the wheels is hardly possible, even carrying the bike would be almost impossible, as you need your hands to hold on or even climb. Only a handful of freaks tackle S5 passages.

In short
TRAIL CONDITIONS: very technical with counterslopes, slippery and not compact terrain. Can include sections similar to high mountain climbing routes.
OBSTACLES: steep ramps, steps in close sequence difficult to overcome
GRADIENT: >>70%
CURVES: very tight bends with obstacles
MTB TECH SKILLS: excellent mastery of trial techniques; shifting the front and rear wheel is very limited.

(*) The Single Trail Skala (or Single Trail Scale – STS) has been developed by Carsten Schymik, David Werner and Harald Philipp. Source here.