While it is called The Rule of Thirds, it is more a rule of thumb as opposed to a hard and fast law that must be obeyed. The rule of thirds says that an image should be divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines. It will look like an imaginary tic-tac-toe board superimposed over the image. The main compositional elements of a photograph should rest along these imaginary lines.
A simple example of the rule of thirds is the horizon within a photograph. The horizon should never be in the middle of the photo. The horizon can rest one third from the bottom of the photo or one third from the top of the photo. The same is true with the vertical lines. Major elements in a photograph should be placed either one third from the left of the image or one third from the right of the image.
The strongest place to put a major element in a photograph is at the intersection of a horizontal and vertical line. These intersections are called power points or crash points. It is aesthetically pleasing to view a picture that has the horizon one third from the bottom and a major element on the horizon one third from the left or right of the image. The horizon could also go at a point one third from the top of the image with the major element intersecting one third from the left or right.
The rule of thirds is used when composing a portrait. The subject's eyes are usually lined up with the top horizontal line. Or for creative shots the nose may be lined up with one of the vertical lines.
Another time to use the rule of thirds is when photographing a symmetrical object. An example of this is a stained-glass dome in an old church. While it is tempting to take a picture with the circular stained-glass pattern in the center of the photo, it is also boring. By placing the center of the dome at one of the intersecting lines, the resulting image will be more visually appealing.
The rule of thirds is probably practiced by more people using photography. However, it is a compositional rule that is used in painting and design as well. The rule of thirds is used in video by placing a person at one of the intersecting points and having the majority of the extra space in front of them.
This is not an unbreakable law. There are times when breaking the rule of thirds adds more creativity to the image. However, it is rare that an image will look better, or naturally pleasing, than when obeying the rule of thirds.
One of the great things about digital photography is that the images are easy to edit on the computer. If the original image was shot poorly-by not obeying the rule of thirds-the image can be fixed by using photo editing software.
By taking notice of photos and paintings that are appealing, it will become obvious that the rule of thirds is used in most good images. A photographer or artist will do well to learn how to compose an image by using the rule of thirds.