The “rule of thirds” is to all intents, a simplified version of an ancient design principle known as the “Golden Mean”. The rule of thirds is not so much a rule as more of a guideline to aid composition, and it is particularly favoured in the field of landscape photography.
To apply the rule of thirds in your composition, you must first imagine* your image divided by horizontal and vertical lines as in the image below. Using these imaginary lines, you can then construct your composition. For example, you can place the focal point of your scene on the where the horizontal and vertical lines intersect or you can place a horizon along the top or lower third of the frame.
Aside from from that fact it produces a more pleasing composition, there are other reasons why sons to place the horizon on either of the third lines. For example, if a sky is bland or lacking in detail then we can place the horizon in the top third and focus the viewers attention away from the sky and onto the foreground. Conversely, placing the horizon on the lower third shifts focus from a bland foreground and towards a more interesting sky as I’ve shown in the image below.
Points of interest
How you arrange the points of interest in your composition can be the difference between a bland and a pleasing final image. We have a natural preference towards objects in a composition that are placed off-centre (see the golden mean). If you look at the image below, you will see that the main point of interest (the tree) is placed off-centre and where the lines of thirds intersect. These points are commonly referred to as “power points” or “power nodes” and to apply the “rule” in your composition, it is simply a matter of placing the main point of interest on one of these points.
*Most modern DSLRs and many compact cameras have a grid that can be switched on and displayed in the LCD