Imagine this scenario (it's happened many times, unfortunately): You have a Dead Bolt Coupler Lock that secures your trailer to the ball portion of your hitch and you leave your truck and trailer parked at the show grounds overnight. When you arrive in the morning to braid your horse, your trailer and the ball portion of your hitch is missing! A thief has merely uncoupled the receiver portion of the hitch, pushed your trailer backward until he can fit a towing vehicle in front of it, and inserted your carrier into the receive of his vehicle! I have a Dead Bolt Receiver Lock that locks both halves of my hitch together, replacing the bolt and cotter pin that the hitch came with. The ball is still easy to remove from the receiver, you just need a key to do so, which means I can leave the ball portion attached to my vehicle even when I'm not towing a trailer. When I am towing, I feel more confident knowing I've done everything I can to make a thief's job difficult. The Dead Bold Receiver Lock  fits all Class 2, 3 & 4 hitches with 5/8" hole.