A six-spoke Geneva mechanism, wikipedia.

Film projectors (as well as film cameras, processing equipment, etc.) use a special mechanism called a Geneva drive to ensure one whole frame is advanced at a time, instead of simply spooling a film continuously. Wikipedia:

The name derives from the device’s earliest application in mechanical watches, Switzerland and Geneva being an important center of watchmaking. The geneva drive is also commonly called a Maltese cross mechanism due to the visual resemblance.

In the most common arrangement, the driven wheel has four slots and thus advances for each rotation of the drive wheel by one step of 90°. If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.

The device itself is beautiful in its simplicity. There are two variations on the drive (external and internal). More at wikipedia.