Film Term:

In 35mm a double reel is 2 single reels joined together, the maximum size being 2,000 feet. Double reels are labeled 1 A/B, 2 A/B etc., to distinguish them from single reels.

Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission