A motor vehicle is a self-propelled road vehicle, commonly wheeled, that does not operate on rails, such as trains or trams. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid electric vehicles and plug-in hybrids. For legal purposes motor vehicles are often identified within a number of vehicle classes including cars, buses, motorcycles, off-road vehicles, light trucks and regular trucks. These classifications vary according to the legal codes of each country.  ISO 3833:1977 is the standard for road vehicles types, terms and definitions. Generally to avoid requiring handicapped persons from having to possess an operator's license to use one, or requiring tags and insurance, powered wheelchairs will be specifically excluded by law from being considered motor vehicles.