A trailer hitch is made up of two parts, a trailer knob and a coupler, or bowl socket. The trailer hitch ball is a metal ball that is mounted on a base. This ball is cast or machined from solid steel, and it usually is completely round, with a flat section on the top. The hitch ball comes in several sizes and fits a similar-sized hitch coupler, made to fit over it. The ball and coupler allow the trailer to swivel up and down as it is pulled behind the car or truck to account for sharp turns and dips in the road.
The more complicated part of a trailer hitch is the coupler. This is shaped like an inverted cup. The cup is shaped like a ball and is mounted on a straight frame or "A" frame. Those frames define the type of coupler it is. When a coupler is attached to the trailer ball, it locks into place. A trigger is pressed and a lever pulled up to release the lock and pull it back. The hitch ball then can be set inside the coupler. The trigger then is pulled and the lever pushed down. The lock inside the coupler snaps forward against the hitch ball holding it in place. Several types of locking mechanisms are available, but the most popular include a single tongue lock that secures the ball. This lock is shaped to fit one sized ball, but some can be adjusted using a mechanism in the coupler similar to that on a pair of vice grips, where the lock is set back a half an inch to accept larger balls.
Other parts of the trailer hitch include the safety chains that are attached to the frame of the coupler. These chains are secured to the trailer. They prevent the trailer from completely detaching from the back of the car in case the coupler lock fails. Additional latch locks also can prevent the coupler from opening. Some use pins that are inserted inside holes in the latch that prevent the latch from coming up. Usually, this is part of the coupler and hitch assembly. The coupler attaches to a hitch post that is mounted to the frame of the car. This hitch post allows the coupler to be removed when not in use.