The filmstrip is a form of still image instructional media, once widely used by educators in primary and secondary schools (K-12) and for corporate presentations (e.g., sales training and new product introductions). It was largely made obsolete by the late 1980s by newer and increasingly lower-cost full-motion videocassettes and later on by DVDs. From the 1920s to the 1980s, filmstrips provided an easy and less expensive alternative to full motion educational films, requiring little storage space and being very quick to rewind for the next use. Filmstrips were durable and rarely needed splicing. They are still used in some areas. Technology A filmstrip is a spool of 35 mm positive film containing a series of images (often thirty to fifty) in sequential order. Like motion picture film, a filmstrip is inserted vertically in front of the projector aperture. The frame size is smaller than normal 35 mm film. Two frames of a filmstrip take up the same amount of space as a single 35mm frame, including its guard band, such that a 25 exposure length of 35mm film could contain up to 50 images. Early celluloid filmstrips were susceptible to combusting like all nitrate-based film. Filmstrips (which often came with an instructor's guide) could be used for either self-paced learning or group presentations. They could be projected onto a wall or conventional screen, or displayed by personal viewing units that contained mirrors and lower-wattage lamps for up-close viewing by one or two people. Filmstrips could be captioned with text, accompanied by a script, or accompanied by a record or tape recording containing narration and, often, musical segments or sound effects. Recordings would contain tones to signal the projectionist to advance the filmstrip to the next frame. Later technical improvements allowed higher-end projectors to advance the film automatically through the use of inaudible recorded tones. Film production By the later part of the 1960s, firms such as Warren Schloat Productions, CBS, The New York Times Company, Scott Education, Coronet Films, Sunburst Media, and Guidance Associates were producing titles featuring photographs by famous artists and of notable events with a synchronized audio track. The music and narration for the filmstrip originally came on a vinyl record. There were filmstrips produced in many different subject areas including music, art, language arts, math, business and home economics. Filmstrips in the science, career, vocational and technical subject areas were produced by such firms as Bergwall