The rule of thirds is a "rule of thumb" or guideline which applies to the process of composing visual images such as designs, films, paintings, and photographs. The guideline proposes that an image should be imagined as divided into nine equal parts by two equally spaced horizontal lines and two equally spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections. Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject.
The photograph to the right demonstrates the application of the rule of thirds. The horizon sits at the horizontal line dividing the lower third of the photo from the upper two-thirds. The tree sits at the intersection of two lines, sometimes called a power point or a crash point. Points of interest in the photo do not have to actually touch one of these lines to take advantage of the rule of thirds. For example, the brightest part of the sky near the horizon where the sun recently set does not fall directly on one of the lines, but does fall near the intersection of two of the lines, close enough to take advantage of the rule.
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. The main reason for observing the rule of thirds is to discourage placement of the subject at the center, or prevent a horizon from appearing to divide the picture in half. Michael Ryan and Melissa Lenos, authors of the book An Introduction to Film Analysis: Technique and Meaning in Narrative Film state that the use of rule of thirds is "favored by cinematographers in their effort to design balanced and unified images" (page 40).
When filming or photographing people, it is common to line the body up to a vertical line and the person's eyes to a horizontal line. If filming a moving subject, the same pattern is often followed, with the majority of the extra room being in front of the person (the way they are moving). Likewise, when photographing a still subject who is not directly facing the camera, the majority of the extra room should be in front of the subject with the vertical line running through their perceived center of mass.