Two film reels with alternating shots — allows optical transitions (dissolves, fades) without visible splice marks. Analog-era standard; now only for 16mm archival work.
When editing film prints — especially in 16mm format — you work with two physical strips of film that you build up in a strict pattern: material for the even cuts on the A-roll, material for the odd cuts on the B-roll. In between: black film strip or spacer. This sounds elaborate because it is elaborate — but this effort pays off as soon as you need dissolves, wipes, or fades.
The practical advantage lies in optical processing. If you have two cut edges directly next to each other on a film strip, you'll see the cut on the editing bench — a visible jump. With A and B rolls, you run both strips through the printing machine simultaneously: while the A-roll is being exposed, the B-roll remains dark, and vice versa. This creates a soft dissolve without two exposed edges ever meeting. You can control the duration and character of the dissolve directly during exposure — via the curve on the printing machine, not afterwards.
Today, you almost only need this system for genuine 16mm archive material or for deliberate format shooting for aesthetic reasons. In a digital workflow, an "A/B roll principle" has long been computer-controlled and invisible — Premiere, Avid, or DaVinci automatically know how to render transitions. Nevertheless, many editors no longer understand it because they never worked with physical film.
If you are still editing analog or juggling archive material: remember that the roll order must be consistent — every cut mark must be precisely punched, every spacer the correct length. A measurement error of a few frames and your dissolve will be crooked. Some editors worked with colored ink markings to avoid losing control. It is manual, but it works reliably, and the result in the print is transparent and reproducible.