The theory is that if you place points of interest in the intersections or along the lines that your photo becomes more balanced and will enable a viewer of the image to interact with it more naturally.
The rule of thirds is applied by aligning a subject with the guide lines and their intersection points, placing the horizon on the top or bottom line, or allowing linear features in the image to flow from section to section.
Studies have shown that when viewing images that people’s eyes usually go to one of the intersection points most naturally rather than the center of the shot – using the rule of thirds works with this natural way of viewing an image rather than working against it.
Using the Rule of Thirds comes naturally to some photographers but for many of us takes a little time and practice for it to become second nature.
In learning how to use the rule of thirds (and then to break it) the most important questions to be asking of yourself are:
What are the points of interest in this shot?
Where am I intentionally placing them?
Rule of thirds can also apply when you are editing images in Photoshop.