minority of jurisdictions it still continues. Classical period (1915–1953) With the advent of feature-length films during the early to mid-teens and the increasing tendency for film screenings to be the main or only event that took place in a purpose-built theater, rather than incorporated into other forms of live entertainment, the role of the projectionist became more specialised and began to incorporate elements of showmanship once again. The safety precautions associated with nitrate required 35mm film prints to be shipped in reels no longer than 1,000 feet (approximately 15 minutes at 16fps). In order show a feature-length film without interruption while the following reel is laced up, two projectors focused on the same screen were used, with the projectionist 'changing over' from one to the other at the end of each reel. 2,000 foot 'double' reels were gradually introduced from the early 1930s onwards (approximately 20 minutes at the standardized sound speed of 24 fps). Until the conversion to sound, electric motors were relatively uncommon on 35mm theater projectors: most were hand-cranked by the projectionist. Contemporary accounts suggest that hand cranking at a consistent speed took a considerable amount of skill. Presentation technique also began to include tasks such as operating auditorium lighting systems [dimmers], curtains [side-tabs] and masking systems and lantern slide projectors. During the 1920s, movie theaters became larger and projection equipment had to adapt to this. Limelight illumination was replaced by the electrically powered carbon arc lamp, and with the arrival of sound electric motors were installed to drive projectors (a more constant speed was required for sound playback than could be achieved by hand cranking). The operation and basic maintenance of audio equipment also became part of the projectionist's job following the introduction of sound. Post-classical period (1953–early 1980s) The technology of cinema projection, and with it the role of the projectionist, changed fundamentally over an extended period between the early 1950s and late 1960s. Nitrate film was superseded by cellulose triacetate for release prints in the US and Europe over about a five-year period following the stock's commercial launch in October 1948 (though older nitrate prints remained in circulation for quite a long time afterwards and are still occasionally utilized in licensed cinemas for special screenings). With nitrate went the restrictions on reel lengths previously necessitated by the fire risk, with the result that systems were developed to enable the projection of a complete feature