Toronto. Plate holders and later film holders became common in the later 1800s and early 1900s to protect sensitized emulsions from unexpected exposure to light. These clever gadgets, which came with a dark slide that could be removed for exposure through the lens, were mounted on the back of the camera (room, etc in Latin). These holders held sensitized glass plates or cut film allowing the photographer to take many exposures by simply removing and exchanging the entire holder.
The dark slide could be removed for exposure, then flipped and reinserted to remind the photographer that the protected sensitized medium had been exposed in the camera. The holder could contain one plate or two depending on construction. Once roll film took off the holders and their dark slides disappeared.
The remaining outpost for many years were the venerable cameras like the Graphic and Graflex lines that continued to use plate holder backs long after roll film was common place.