Christopher Jacobs wrote:One reel of 35mm film was originally standardized at 1000 feet by the late 1890s, although by the mid-1910s many if not most theatres had projectors that would hold 2000-foot reels (still the standard for theatres that use a pair of projectors with changeovers instead of one projector with the whole film wound onto a platter). Thus a "three-reeler" would look like one-and-a-half reels if mounted on the double-size 2000-foot reels. If you found a "three-reeler" on three full 2000-foot reels, it would technically be referred to as a six-reeler (mounted on three double-size reels) and would run about an hour at 24 fps and could run close to an hour and a half if from the 1910s and intended to run closer to 16 or 18 fps. Also note that in the early 1910s, a three-reeler could well be considered a "feature," as the slower projection speed at that time would make three reels last 40-50 minutes instead of the half-hour or so they'd run at 24 fps.
I'm going to respectfully disagree with Chris on some of the points above.
1) A "reel" by definition in the motion picture industry is a 1,000 foot reel, which may opntain less than 1,000 feet of 35mm film--though sometime a given film mught have slightly more than 1,000 feet in a single reel. Irvin Willat, who supervised editorial for Thomas Ince before he became a director said that in general footage on a reel was kept to 980 feet or less so as not to overfiill a reel.
2) Although some projectors were equipped to handle larger loads, the standard of "double reels" (or 2,000 footers) for projection was not formalized in the industry until 1937. So, a film like "Gone With The Wind" is a 24-reeler, though because it was released after 1937 it shipped on (for various technical reasons) something like 16 double reels. But, and this gets a little complicated, Double reel # 1 would consist of reels one and two (as some labs/studios would designate) OR Reel 1A and Reel 1B as others would designate. These reels were always spliced together in "positive assembly" at the lab or the film exchange before shipping--after 1937. A film like "It Happened One Night" (1934) would have shipped on 10 single reels at the time of its initial release. At the time of its 1948 reissue, it would have shipped on 5 double reels, but it still would have been designated as a 10-reel picture.
3) The reasons for maintaining the 1,000 reels long after double-reel projection became standard had a lot to do with A) storage--vaults were generally set up to handle 1,000 reels; B) Ease of working--it is a lot tougher for an editor, sound editor, negative cutter or lab technician to hand wind ganged 2,000 foot reels than 1,000 reels; and C) tradition
4) It is really only in recent years--maybe the last 10 of 15 that--again largely for technological reasosns--that labs have routinely joined the A & B negative reels nefore printing. Optical sound tracks are now also recorded in 2,000 foot lengths. So today, and for the past several years--but not that long--it has become more common to refer to a 10,000 foot film as a 5-reeler (because of the printing units in which the negative is mounted) rather than a 10 reeler. The real change occurred primarily with the switch from film based to digital based sound editing technologies because it fanally became practical for sound editors to assemble their work in the longer "double-reel" format.