To oversimplify the rule of thirds, it dictates that your subject shouldn’t be centered in the frame. In addition it shows you how to best compose your photographs.
The slightly more-complex explanation is that you should divide your field of view into 9 equal quadrants—3×3, by placing two equally spaced lines vertically and another to equally spaced lines horizontally on your field of sight. This 9-quadrant grid will help you create more dynamic photos.
The elements in your photo should line up with these lines or the lines’ intersections (called power points or crash points). If you use primarily the outside quadrants for your photos’ subjects experts say you will have more energy and tension; it will also ensure your subjects stay off-center. Using the rule of thirds means that your photos will have its primary elements closer to the outer edges. To put it another way, the main elements will feel loaded more to one side of the photo than the other–off center.
For example, if you are taking a photo of a lake at sunset you would ideally want the water’s edge of the lake to follow the bottom line of your grid. If you are taking a picture of a flower you would want the center of the flower to fall on one of the outer lines, and not in the center quadrant. When taking pictures of people you line their bodies up with the vertical lines—unless they horizontal but then that is a whole different type of photography.
Once you have mastered the use of the quadrants and lines you can begin to play with your elements’ composition—as the rule of thirds is more of a guideline than a hard and fast photographic law.
Eventually this rule will become part of your photography and you will begin to see the world through the rule of thirds. When you are a rule-of-thirds master you will see naturally envision elements within the grid throughout regular life. You will also begin to see the flow of element’s lines and understand how they should be properly positioned in a photograph. It will just click into place.
For photography buffs who are also history buffs—the rule of third has a long lineage dating back to 1797. John Thomas Smith officially coined the term in his book Remarks on Rural Scenery. Interestingly enough this was before the first photograph was ever taken, in 1817.