Arri PL is a lens mount developed by Arri for use with both 16 mm and 35 mm movie cameras. The PL stands for "positive lock". It is the successor mount to the Arri bayonet; however, unlike the bayonet mount, it is incompatible with older Arri-mount lenses, due to the larger diameter. (This can be rectified with an adapter relatively easily, as the flange focal distance is identical.)
The mount contains four pronged flanges which each contain a notch towards the center. Any of these four notches can be used to align the mount to a locating pin located approximately 45 degrees clockwise from the top of the camera's lens mount. This means that the lens can be oriented in any of four different configurations, which may be decided by factors such as focus puller position, camera position, and preference to working with meters or feet (if a lens has different line indicators on each side). The mount is locked into place using a friction locking ring which, in conjunction with the four prongs of the flange, creates a very strong lens seating. This has become a crucial factor in recent years, as bigger lenses with zoom capabilities, longer focal lengths, or larger lens elements have increased the need for mount stability due to the greater stress placed upon the mount by these devices.
Furthermore, the tendency is for flange focal distance to become a more critical factor as sharper lenses, film stock with higher resolving power, and post-production workflows which preserve more of the original camera negative's information continue to make technological progress and advancement. Flange focal distance is also more likely to be rigorously checked as tastes for wider angle lenses continue to push the envelope, as wider lenses have a much narrower tolerance for lens displacement.
Because of all of these reasons, the Arri PL mount has been extremely successful since it was first released at photokina 1982, and has been adopted by virtually all professional movie camera manufacturers since that time, with the notable exception of Panavision, whose PV mount remains the only other lens mount still in major usage by a large number of professional productions.