Highway rigs, also known as SEMIS (5 letters), are large trucks used to transport goods and materials on highways. They are typically composed of a tractor unit and one or more trailers, and are often used for long-distance hauls. The tractor unit is usually powered by a diesel engine, and the trailers are connected to the tractor unit via a fifth wheel coupling. Highway rigs are an important part of the transportation industry, as they are used to move goods and materials across the country.