A filmstrip is a spooled roll of 35 mm positive film with approximately thirty to fifty images arranged in sequential order. Like 16 mm film, a filmstrip was inserted vertically down in front of the 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 50 filmstrip images. Early celluloid filmstrips had a habit of melting or combusting from the intense and sustained heat of the projection lamp. These were called pictural filmstrips, the first filmstrips that were produced in a complete set.