Shot on threes (ones, twos, etc.)
Motion Picture film runs 24 frames per second, meaning that to get full, fluid, motion-picture quailty action, animators would have to draw 24 cels for every second of film. This is an expense that most anime budgets can't handle (unless your name is Hayao or Osamu or Mamoru), and so shortcuts are taken in almost all cel-animated TV shows, many OAVs and even a few movies. Often TV shows are "shot on threes": every cel is held for three frames instead of one (i.e. the number of cels is cut in three). A show shot on threes would have 8fps animation, in effect. Depending of how much motion is in the scene, a show shot on threes really isn't all that noticably different from a show shot with full motion, and so is useful as a budget move. American TV animation is usually shot on twos and so is more fluid than anime, but to compensate for the higher cost they must sacrifice details. If a scene has little motion, it is possible to shoot it on fours, fives and more, as illustrated by some static talking scenes where the only thing moving is the mouth. But high-action scenes may have to be shot on twos or ones in order to adequately convey the sense of motion. "Shot on threes" is more an average than a constant rule.