In film projection, 24 frames per second is the standard speed for most theatrical motion pictures.
Early films had no universal frame rate. Silent-film cameras and projectors were often hand-cranked, so speeds could vary substantially, commonly ranging from roughly 16 to 20 frames per second. The arrival of synchronized sound required a steady playback speed so dialogue, music, and sound effects would remain at the correct pitch.
The industry settled on 24 frames per second in the late 1920s because it provided acceptable sound quality while conserving expensive film stock. A faster rate could improve motion smoothness, but it would also consume more film. The standard therefore became a technical and economic compromise.
Twenty-four frames per second does not mean every moving image uses that rate. Television, digital video, animation, and high-frame-rate formats may use 25, 30, 48, 50, or 60 frames per second. In traditional film projection, each frame is often flashed twice or more to reduce visible flicker, even though the film itself advances at 24 frames per second.