To understand frame hardware you should know that there are several types of hardside waterbed frames. There are three basic variations:
-4 Board Extended Frame with Headboard
4 Board Frame:
The 4 boards are the two side rails, the headboard and the footboard. Notice that the headboard and footboard are different. The headboard sits between the side rails. The footboard is at the end of the footboard.
You should always use duct tape or similar tape to cover any hardware inside your bed to prevent chafing against the liner or mattress. This is a leading cause of premature mattress failure.
5 Board Frame
4 Board Extended Frame with Headboard:
Deck and Footboard Detail View from front-right side of bed
Notice in the above illustration that the footboard has a groove milled so that it fits over the deck. The headboard is done in a similar manner. In this way the frame is supported at the top and bottom.