In 1933, South Carolina passed a regulation prohibiting the operation on its state highways of certain motor trucks and “semi-trailer” motor trucks which exceeded width and weight limitations. For purposes of the weight limit, the statute considered semi-trailer motor trucks, which were composed of two pieces, as one single unit. Barnwell Bros., Inc. (plaintiff), an out-of-state trucking company, brought suit in district court against the South Carolina Highway Dept. (defendant), challenging the South Carolina weight restrictions as an impermissible restriction on interstate commerce. The district court upheld the South Carolina regulations on the grounds that Congress had not acted to regulate this area of interstate commerce, thus leaving all regulation up to the individual states. The court of appeals reversed. The South Carolina Highway Dept. appealed to the United States Supreme Court.