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. The gooseneck tapers forward from the trailer body to form the nose. A gusset on each side strengthens the joint. The coupler, attached to the neck or tongue, hitches the trailer to a towing vehicle. A coupler has a rating capacity of the maximum weight it should pull, which can differ from the trailer's rating capacity and the capacity of the trailer hitch ball.