The negative of an edited film, cut to correspond to picture, built into 2 rolls, A and B, to allow for invisible splices, instant changes of the timing lights and fades and dissolves without the need for opticals. The A roll will have all the odd numbered shots, with black leader in place of all the missing shots. The B roll will have all the even numbered shots, with black leader in place of all the shots on the A roll. The negative is printed in three passes through the contact printer, one for each roll and another for the soundtrack. Only after all the elements have been exposed onto the print stock is the print developed.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission