The "line of action" is a simple, usually curving, line that travels through all the forms of a pose. A Disney animator, possibly Bill Tytla or Art Babbitt, used an S-shaped line passing through the pose of this character model drawing of Geppetto from Disney's Pinocchio.
Other artists have applied the principle, including the cartoonist T. S. Sullivant (1854-1926), who was a big influence on the Disney animators.
Here's another example from a Victorian painter, Herbert Draper (1863-1920), in his canvas "Flying Fish."
Feel free to leave links of other examples in the comments.
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