Used both for making home movies and for showing shortened "cinema-at-home" versions of professionally made feature films, it enjoyed popularity for several decades in Europe, including the UK, but was virtually unknown in the US. Of importance in making motion picture film practical for home use was the manufacturing option of safety film in the 1920s. The nitrate film used by professionals required caution in handling and projection because it is highly flammable. Nitrate film badly stored has been known to spontaneously combust. The 16 mm format, which used only safety film, was introduced by Eastman Kodak in 1923 and became a standard in the non-professional market. Although 16 mm had the advantage that users were not tied to one equipment manufacturer, and there were obvious cost advantages compared to standard 35 mm, the advent of an even smaller and cheaper format ultimately relegated 16 mm to professional users, particularly in the educational market. In 1932, Kodak introduced another new format, 8 mm, now called "Standard 8" or "Regular 8", which put four frames into the area occupied by one standard 16 mm frame. The film usually came in 16 mm wide "Double 8" form, which ran through the camera in two passes (one in each direction) and was slit in half after processing. The "Straight 8" variant came already cut to 8 mm width. In either case, the amount of film stock used per frame was again reduced by 75%. This finally brought home movies within the reach of the average family. The smaller format also made possible smaller and more portable cameras and projectors. The introduction of Kodachrome color reversal film for 16 mm in 1935, and for 8 mm in 1936, facilitated home color cinematography. The availability of reversal film, both black-and-white and Kodachrome, was very important to the economics of home movie-making because it avoided the expense of separate negatives and positive prints. The original 8 mm format was largely superseded within a few years of Kodak's 1965 introduction of Super 8 film. The Super 8 format used the same film width as standard 8 mm, but the perforations were smaller, making room for larger frames that yielded a clearer image. In addition, Super 8 film came in cartridges for easier loading into the camera. High-end Super 8 also could be purchased with a magnetic audio track, allowing for synchronized sound home movies. Single-8, a competing product from Fujifilm, was also introduced in 1965.