The most common instance of film holder is the sheet film holder. Also referred to as a "dark slide" or "double dark slide", they are flat devices, slightly larger than the films they hold, which commonly hold one sheet of film on each side. The plate holder, which is a very similar device, holds glass plates instead of sheet film. A dark slide, from which the device derives its alternate name, is simply a dark cover that slides into a recess in the holder to protect the film (or plate) from exposure to light. Many dark slides have differently colored bands or handles on each side, one usually light and the other dark, so the photographer can distinguish between exposed and unexposed film.