It's funny how things seem to come in clusters. Steve Worth posted the entire set of bar sheets for the early Merrie Melody cartoon Shuffle Off to Buffalo (generously provided by Mark Kausler). Mike Sporn kicked in some bar sheets on his blog from the books Techniques of Film Animation and Animation Art in the Commercial Film. Kevin Langley put up some MGM bar sheets for Tee for Two and Officer Pooch, two Hanna-Barbera cartoons.
Because the marriage of picture and sound was one of the main selling points of cartoons in the early '30's, directors had to deal with musical beats in order to make the films work. Anyone who wants to try and puzzle out the approach finally has enough examples available to work with.
Actions and cuts were all placed on the beat in order to make the cartoon flow. Even if the music was not chosen before the animation, the director would still determine the beat so that when music was composed it would fit.
I know at Warner Bros. they had exposure sheets printed up that were marked for 8, 10 and 12 frame beats.
Which brings me to my metronome. People generally know that animators use stopwatches to figure out timing, but I never found them particularly useful. I needed something with smaller increments. I defy anybody with a stop watch to figure out the difference between 12 and 14 frames.
With a metronome, it's easy. And while you're listening to the metronome click away, you can be moving your finger or pencil over the page to simulate the action and get a feel for how long something takes.
At 24 frames per second, here's the relationship between frames per beat and beats per minute:
24 beat = 60 beats per minute
22 beat = 65 bpm
20 beat = 72 bpm
18 beat = 80 bpm
16 beat = 90 bpm
14 beat = 103 bpm
12 beat = 120 bpm
10 beat = 144 bpm
8 beat = 180 bpm
6 beat = 240 bpm
The formula is 60 divided by (frames-per-beat divided by 24) equals beats per minute.
When I was directing, I left the sheets blank during dialogue, since the length was predetermined and I wanted to give the animators some freedom. But whenever we got into action, I timed out everything. If a character was walking or running, I established the beat based on the metronome. I would determine how long an anticipation would last and how long an action would take. It was the only way to set a pace for a series of shots. As I could not guarantee that I would give a whole sequence to one animator, I had to establish the timing so if several animators did a sequence the pace would be consistent overall.
I don't think that I could animate or direct without a metronome. It was great if I could establish a musical beat that a composer would follow, but even if music was going to be slapped on, I was controlling the pacing of the action before any animation got done. It saved me time and gave me a predictable result.
Directors and animators who worked on cartoons timed on bar sheets came to know how things would look before they saw anything on screen. With computers, there's a tendency to do a bunch of board panels or poses and start sliding them around on a timeline to determine pacing. I don't know if people who do that develop a sense of timing, as they have to see the result before they know if it works.