Imagine that your image is divided into 9 equal segments by 2 vertical and 2 horizontal lines. The rule of thirds says that you should position the most important elements in your scene along these lines, or at the points where they intersect.
Doing so will add balance and interest to your photo. Some cameras even offer an option to superimpose a rule of thirds grid over the LCD screen, making it even easier to use.
What is the Rule of Thirds?
The rule of thirds involves mentally dividing up your image using 2 horizontal lines and 2 vertical lines, as shown below. You then position the important elements in your scene along those lines, or at the points where they meet.
The idea is that an off-centre composition is more pleasing to the eye and looks more natural than one where the subject is placed right in the middle of the frame. It also encourages you to make creative use of negative space, the empty areas around your subject.
How to Use the Rule of Thirds
When framing a photo, imagine the scene divided up as above. Think about what elements of the photo are most important, and try to position them at or near the lines and intersections of the grid. They don't have to be perfectly lined up as long as they're close.
You may need to move around to get the best composition. This forces you to think more carefully about the shot, and is a good habit to get into whether you're using the rule of thirds or not.
To help you out, some cameras have a setting which overlays a rule of thirds grid onto your photo. This removes all guesswork and helps you get your positioning even more accurate.
The rule of thirds is very versatile and can be used on any subject. Below are some example of it being used effectively in different types of shots.
In landscape shots, it's common to position the horizon along the centre of the frame, but this can give the photo a "split in two" feel. Instead, place it along one of the horizontal lines.
Try to include another interesting object, such as the tree in the photo above, and position it according to the rule of thirds. This provides an "anchor", a natural focal point for the scene.
It's a good idea to position people off to one side of the frame. This provides some "breathing space", shows the subject's environment, and stops the photo from looking like a mugshot.
We are naturally drawn to people's eyes. Place them at one of the intersections on the rule of thirds grid to give the shot a clear focal point.
Here the main subject has been placed at one of the intersections, and also along one a vertical line. The twig roughly follows the top horizontal line. The empty space at the bottom left provides balance and prevents the picture from feeling overcrowded.
Vertical subjects such as this lighthouse can split a photo in two, in much the same way as a horizon can do horizontally. To avoid this, position them off-centre in your composition.
When photographing moving subjects, position them as normal, but also pay attention to the direction they're moving. As a general rule you should leave more space in front of them than behind, to show where they're going.
Breaking the Rule
As with all rules (at least in photography), the rule of thirds doesn't apply in every situation, and sometimes breaking it can result in a much more eye-catching, interesting photo. Experiment and test out different compositions even if they go against any "rules" you've learned.
However, learn to use the rule of thirds effectively before you try to break it - that way you can be sure you're doing so in order to get a better composition, rather than just for the sake of it.