“Animation is not the art of drawings that move but the art of movements that are drawn.”
In traditional hand-drawn animation, a lead animator draws key drawings in a scene. These are the most important actions and represent the extremes of the action. The idea is to provide enough detail to present the main elements of movement. Next, the animator decides how long each action should last on the screen. After this is done, these key drawings are passed to assistant animators who draw the inbetween frames to achieve smooth movement. The action duration dictates how many inbetween frames are needed. This process is called tweening, which is short for inbetweening.