Most watch cases have a separate, removable back to allow access to the movement. There are multiple types of case backs.
A screw-down or threaded case back has threads that allow a tight, often waterproof seal. There are various methods of removing a screw-on case back employed by different manufactures. Some have a flat-sided parallelogram on the back, others use smaller square or scalloped notches, and Rolex has a patented threaded design.
Most early watches used a simple snap-on back. This is held to the case with pressure. Such watches often feature an indentation along the case back so a tool can be inserted to pry off the back.
If the back of a watch case is held on by screws, it is called a screw-on or screwed back.
Supercompressor cases have a bayonet style back similar to a screw-down back but with an indentation at the end of the screw travel so the case back is always attached with the same pressure.
Early pocket watches often featured a simple hinged dust cover over the movement. This has been retained in some watches in association with a sapphire caseback.
Display Case Back
Today, many watches have a piece of sapphire glass in the case back, allowing the movement to be viewed. This is in addition to one of the above case back attachment mechanisms.
Bovet was one of the first manufacturers to produce a display case back, equipping some of their "Chinese Watches" with a pane of glass so the movement could be viewed as early as the 1820's.