There are several different types of trailer coupler locks. In addition to the McPhail lock, there is the wedge lock and the latch lock. The wedge and latch locks work in a similar fashion, with the difference being in which side rail they fit on.
The wedge lock fits into the side rail on the truck, while the latch lock fits into the side rail on the trailer.
The most common type of lock is the McPhail lock, which fits over the head of the kingpin and locks onto one of the slots at the end of each side rail.
Another type, the Dee Zee DZ433, features a rotating steel bar with notches and a keyed cylinder lock with dual locking levers.