Article excerpt written by John Lasseter, Pixar, San Rafael, California
ACM Computer Graphics, Volume 21, Number 4, July 1987
PRINCIPLES OF TRADITIONAL ANIMATION
APPLIED TO 3D COMPUTER ANIMATION
Between the late 1920’s and the late 1930’s animation grew from a novelty to an art form at the Walt Disney Studio. With every picture, actions became more convincing, and characters were emerging as true personalities. Audiences were enthusiastic and many of the animators were satisfied, however it was clear to Walt Disney that the level of animation and existing character were not adequate to pursue new story lines– characters were limited to certain types of action and, audience acceptance notwithstanding, they were not appealing to the eye. It was apparent to Walt Disney that no one could successfully animate a humanized figure or a life-like animal; a new drawing approach was necessary to improve the level of animation exemplified by the Three Little Pigs. 
Disney set up drawing classes for his animators at the Chouinard Art Institute in Los Angeles under instructor Don Graham. When the classes were started, most of the animators were drawing using the old cartoon formula of standardized shapes, sizes, actions, and gestures, with little or no reference to nature.  Out of these classes grew a way of drawing moving human figures and animals. The students studied models in the motion  as well as live action film, playing certain actions over and over.  The analysis of action became important to the development of animation.
Some of the animators began to apply the lessons of these classes to production animation, which became more sophisticated and realistic. The animators continually searched for better ways to communicate to on another the ideas learned from these lessons. Gradually, procedures were isolated and named, analyzed and perfected, and new artists were taught these practices as rules of the trade.  They became the fundamental principles of traditional animation:
1. Squash and Stretch — Defining the rigidity and mass of an object by distorting its shape during an action.
2. Timing — Spacing actions to define the weight and size of objects and the personality of character.
3. Anticipation — The preparation for an action.
4. Staging — Presenting an idea so that it is unmistakably clear.
5. Follow Through and Overlapping Action — The termination of an action and establishing it relationship to the next action.
6. Straight Ahead Action and Pose-To-Pose Action — The two contrasting approaches to the creation of movement.
7. Slow In and Out — The spacing of the inbetween frames to achieve subtlety of timing and movement.
8. Arcs — The visual path of action for natural movement.
9. Exaggeration — Accentuating the essence of an idea via the design and the action.
10. Secondary Action — The action of an object resulting from another action.
11. Appeal — Creating a design or an action that the audience enjoys watching.
The application of some of these principles mean the same regardless of the medium of animation. 2D hand drawn animation deals with a sequence of two dimensional drawings that simulate motion. 3D animation involves creating a three dimensional model in the computer. Motion is achieved by setting keyframe poses and having the computer generate the inbetween frames. Timing, anticipation, staging, follow through, overlap, exaggeration, and secondary action apply in the same way for both types of animation. While the meaning of squash and stretch, slow in and out, arcs, appeal, straight ahead action, and pose-to-pose action remain the same, their application changes due to the difference in medium.