The term double system refers to sound and picture as two separate elements, recorded, edited or projected in sync. 16mm and 35mm use the double system format. A camera photographs the picture and a tape recorder records the sound. In the end, the final print is single system, combining sound and picture onto the same piece of print stock.

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