OK, technically the coupler is part of the trailer -- the piece located at the end of the tongue -- but we're including it as a hitch accessory because you won't be able to hitch your trailer to your vehicle without a coupler. Like hitch balls and ball mounts, couplers are rated by their weight capacity. A coupler must be able to accommodate the gross trailer weight, or GTW. GTW refers to the total weight of a trailer, including all of its contents.
Couplers come in two basic types: the A-frame coupler and the straight-tongue coupler. The former fits on trailers with A-shaped tongues; the latter fits on trailers with straight tongues. In either design, a hollow cup fits over the hitch ball, so it's critical that the diameter of the coupler matches the diameter of the ball. Once the coupler is seated on the hitch ball, a spring-loaded latch locks the two pieces together.
But the more mechanisms keeping the tow vehicle and the payload secure, the better right? That's the theory behind the next hitch accessory on our list.