Polaroid cameras can be classified by the type of film they use. The earliest Polaroids (pre-1963) used instant roll film, which has since been discontinued. Roll film came in two rolls (positive/developing agent and negative) which were loaded into the camera and eventually offered in three sizes (40, 30, and 20 series). Later cameras utilized "pack film," which required the photographer to pull the film out of the camera for development, then peel apart the positive from the negative at the end of the developing process. Pack film initially was offered in a rectangular format (100 series), then in square format (80 series). Later Polaroids, like the once popular SX-70, used a square format integral film, in which all components of the film (negative, developer, fixer, etc.) were containe