It is said that the trailer coupler lock was invented by Byrel Leigh "B. L." McPhail, an employee of the Yellow Freight System, Inc. (now YRC Worldwide) in Boulder City, Nevada. He was not given credit for it at first, but later the company awarded him a patent for it.
The purpose of the invention was to prevent trailers from being accidentally uncoupled. McPhail designed a locking device that would fit over the coupling head and lock it, preventing the coupler from releasing. McPhail also invented other devices for use with trailers, such as a hydraulic brake assist and a hydraulic lift axle.
An invention of such impact was the result of an accident in which McPhail's truck separated from the trailer while on the road and got hit by another vehicle. In the accident he lost several teeth and injured his eyes. While he was recovering from surgery, he had time to think about how he could have prevented the accident.
The trailer coupler lock was patented in 1967 and is still in use today. While there are many different types of trailer locks available on the market, the McPhail lock is one of the most common.