Filmstrips were a form of multimedia usually used for training or educational purposes, combining a sound recording with a strip of still images on 35 mm film.
From the 1940s to 1980s, filmstrips provided an easy and inexpensive alternative to 16mm projector educational films, requiring very little storage space and being very quick to rewind for the next use.
Like 16 mm film, a filmstrip was inserted vertically down in front of the filmstrip projector aperture, rather than horizontally as in a slide projector. Therefore, the frame size is smaller than normal 35 mm film. Two image frames of a filmstrip take up the same amount of space as a single 35mm frame, including its guard band, so that a 25 exposure 35mm film can contain fifty filmstrip images.
Typically, a filmstrip’s running time was between ten and twenty minutes and narration was provided by a 33⅓ rpm record, or from the early 1970s, a Compact Cassette tape. At the appropriate point, a tone would sound, signalling the instructor to advance to the next frame. In the 1970s, filmstrips projectors were produced that could automatically advance the film when a 50 Hz subaudible tone recorded on the tape was detected. On some tapes, one side would have audible tones for the older projectors, and the other side would have the subaudible tones for the newer automatic projectors, while others combined both types.
By the 1980s, the increasing affordability of video players meant a decline in filmstrip use.