The lower part of the case, fixed beneath the case middle. The case, either in metal or in sapphire crystal to reveal the movement, may be of the screw-down, snap-on, screw-on or hinged dust cover type.
- Screw-down back : the back and the case middle are respectively fitted with a screw thread and thus screwed to each other.
- Snap-on back : the back is held on to the case middle by pressure.
- Screw-on back the back is held on to the case middle by four screws.
- Hinged dust cover : the sapphire crystal is held on to the case middle by pressure, and the dust cover is hinged to the case middle, enabling one to reveal the movement.
On Patek Philippe watches, the back may also be integrated with the case middle. On those kind of models, the watch is equipped with a movement with a “split stem”.