The rule of thirds is a compositional rule of thumb in photography.
The rule states that an image can be divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines. The four points formed by the intersections of these lines can be used to align features in the photograph. Proponents of this technique claim that aligning a photograph with these points creates more tension, energy and interest in the photo than simply centering the feature would.

The application of the rule of thirds to photographs is considered by many to make them more aesthetic and professional. The rule of thirds can be applied by lining up subjects with the guiding lines, placing the horizon on the top or bottom line instead of the center, or allowing the subject(s) to flow from section to section, as is done with the example photograph.
Source: wikipedia
Used: TECHtata01: Rule of Thirds
Example uses:
An interesting link: Photoshop Plugin for the various systems based on the golden proportion.
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