Fulton Coupler, Ball Size 2In, 2-1/2In Tongue Width, Zinc Finish, Rating 3,500 Lbs., Trailer Couplers A coupler is the connection point mounted on the trailer which connects the trailer to the vehicle. When replacing a coupler there a few things that you have to know. First, what is the hitch ball size for the coupler. Secondly, what type of trailer do you have (Straight Tongue or A-Frame)? Lastly, what is the Gross Trailer Weight of the Trailer. These are the basic questions for finding the correct coupler replacement. Fulton couplers are of the highest quality steel construction featuring: Adjustable lock nut for proper ball tension, Locking hole in coupler handle and Positive locking trigger.