A curved line is dynamic, ever changing. It has more contrast than a straight line and it is more naturalistic.
The straight line is more monotonous, has less contrast, and is more static in character. But the straight line is more exact, so architects use the straight line to simplify building requirements.
The dynamic qualities of the curved line can be combined with the decorative qualities of the straight line by using a curved line with a continuous ratio so that it becomes repetitive and, therefore, decorative. The spiral is such a curved line, a favorite of wrought iron workers as well as great painters like Gauguin and Matisse.
Putting horizontal lines in the bottom of a picture, vertical lines in the middle, and diagonal lines at the top increases the decorative quality of line by its multiple, repeated use.
If we use line in the manner illustrated, we get horizontal lines pushing the front plane of a picture back, vertical lines in the middle plane holding the middle plane in the middle, and diagonal lines in the background move the background forward. In other words, the repeated use of line in this fashion flattens the picture plane, creating a more decorative picture.
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