What is Key Frames and Inbetweens

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The principle of timing is tied very closely with the principle pose to pose.

If these diagrams are time lines, and the numbers in the circles are key frames or frames drawn in the pose to pose technique then the numbers outside of the circles are inbetweens.

The top timeline would involve movements that are in a very rhythmic pattern, which would be a very stale animation. Whilst straight ahead animation is spontaneous, it is hard to judge what the timings are are whether it is becoming stuck within a rhythmic pattern.
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The bottom timeline would involve movements that are timed differently throughout. It is easy to plan this within the technique of pose to pose, once the key frames are set out it is easy to set the timings of the inbetweens.