RTV (Road Taxed Vehicle) trialing is the most common form of trialing. As the name suggests, it is for vehicles that are road-legal (and thus required to pay road tax). This excludes vehicles that are highly modified or specially built. RTV-class vehicles can carry a wide range of suspension modifications, as well as off-road tires (provided they are road-legal), recovery winches, raised air intakes etc. Vehicles on RTV trials are usually best described as "modified from standard"—they use the standard chassis, drive-train and body that the vehicle was built with. Whilst modification is not necessarily required for an RTV trial, at the very least the vehicle would be expected to have some under-body protection such as a sump guard, differential guard and solid sills. RTV courses are intended to be non-damaging and driven at little more than a walking pace and a course properly laid out would be drivable without damage. However, the terrain usually includes steep slopes, water, side-slopes, deep ruts and other obstacles that could potentially damage a vehicle if mistakes are made or poor driving technique is used, and vehicle modifications increase the chance of success.