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.
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.
Wristwatches with display casebacks first appeared in the 1950s, with Eastern Jeweller and Watchmaker commenting in 1952 that the Basle Fair included “a number of men's wrist watches … made with transparent backs through which the highly finished, jeweled-lever movement can be seen.” This coincided with the development of automatic winding, with customers eager to see and show off this new development. Another popular complication at the time was the mystery dial, which also included transparent elements. This use of transparent materials was known as “skeleton” at the time, though that term would refer to open-worked movements in future decades.
Transparent case backs remained fairly rare throughout the 1950s and 1960s, however, and were generally only used when a novel movement was used. The Bulova Accutron Spaceview used a transparent case, for example, as did some early quartz watches. Perhaps the most notable transparent case was the shaped crystal case used on the original Corum Golden Bridge in 1979, though the back of that case was solid. Transparent cases were fitted to early automatic quartz models as well, including the Jean d'Eve Samara and Seiko AGS Quartz of 1988.
By this time, great advances had been made in synthetic sapphire and hardened glass. Previously, scratch-prone plastic was used for transparent backs, but many manufacturers were willing to experiment with glass and sapphire display case backs. This practice coincided with the resurgence of mechanical watches in the 1990s and the development of haute horology.
Today, most luxury watches include transparent display case backs. Notable hold-outs include Rolex, which has only rarely equipped a watch with such a back, and sports models.