Repetition, i.e., the use of similarly shaped elements, is a strong unifying force because of the Gestalt grouping principle. Two similar shapes, even at different sides of an image, will lead the eye from one to the other. A shape can obtain more visual weight by being repeated, e.g., by its own shadow.
Rhythm is based on repetition, but requires more similar elements than does repetition. A small number of similar elements will be perceived as a single group, but rhythm requires enough similar elements to be perceived as several related groups. The repetitive groups must have some variation to achieve rhythm. The most common way to achieve rhythm is with common shapes. Rhythm can evoke an emotional response, e.g., curvilinear forms can calm whereas angular lines can stimulate.
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