Rolleiflex and Rolleicord cameras were fitted with bayonet mounts for lens accessories (caps, hoods, filters and close-up lenses) from about 1937. Franke & Heidecke's patent for the mount noted that bayonet mounts had been used previously, but that these tended either not to allow two accessories to be mounted together, or to require them to be stacked, so that each could not be fitted and removed independently. The Rollei bayonet mount allows a hood and a filter to mount independently, the hood fitting outside the mount, and the filter inside it.
The bayonet was soon made in several sizes:
- Bay I (universally-used abbreviation for "bayonet type I") is the mount provided on cameras with the f/3.5 Tessar, Triotar and Xenar;
- Bay II is the mount for the f/3.5 Planar and Xenotar;
- Bay III is the mount for the f/2.8 Planar and Xenotar; it was also used for the Tele-Rolleiflex with the 135 mm f/4 Sonnar.
- Bay IV is the mount used for the Wide-angle Rolleiflex with the 55 mm f/4 Distagon.
Bay VI is the bayonet accessory mount used on lenses for the SL66 SLR camera.