The rule of thirds states that you should place your subjects and other important compositional elements along the division lines between the nine squares formed by dividing both vertical and horizontal edges into thirds.
A standing subject would be placed on one of the two vertical lines, and the horizon would be placed on one of the two longitudinal lines. A subject can be aligned either through its center or along one of its edges. The most important compositional elements would be placed on the four points created by the intersection of these lines. Bear in mind that placing subjects along all four lines runs the risk of a static composition with symmetry from top to bottom and left to right.
Of course, there are many great photographs and other visual compositions throughout history that do not conform to this. Nonetheless, the rule of thirds is a good start for anyone who is learning composition, and this knowledge can help you produce satisfying images at any stage in your career.
Also see: Golden Mean
Also see: Comparing Thirds to Golden Mean
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