The trailer is a combination of stable structures and moving parts. In the simplest terms, a trailer is a box secured to a chassis. Trailers are designed and built from the floor up. A subframe's structural members form the trailer's integrity. These beams run perpendicular to the trailer's sides, or bottom rails.
Rails form the frame's bottom and top.
Typically, the subframe and frame are welded of square metal tubing in a post and rail construction. Posts are concealed by the wall panels, or sidewalls. The rounded design helps the trailer move more easily through air. Some models feature a streamlined, aerodynamic V-shaped nose. A trailer is pulled through either a neck, as in the gooseneck shape, or through a tongue, which is a V-shaped section on the bumper pull trailer.