It must relate back to some intensely rebellious incident in my childhood (one of a trillion, no doubt), but I've never been very good at following (or even accepting) rules. That's probably why I balk abruptly when I hear anything--even if it's beneficial idea--labeled as a "rule." Still, when it comes to photographic composition, there are some guidelines that can be very useful in speeding up and improving the image-design process and one of those is commonly known as the rule of thirds. I like to think that perhaps the word "rule" is used here to indicate "ruling" (as with a ruler) rather than a "rule" (a convention not to be broken). OK, enough semantics.
You've probably heard or read about the rule of thirds many times and even if you haven't, you've almost certainly invoked it without even knowing it. For many visual artists it's such a natural concept that we use it without even thinking about it. Essentially what the rule suggests (not dictates) is that you divide the frame, both horizontally and vertically, using a set of invisible dividing lines. Using this
design principle will help you establish a more harmonious sense of balance in your images.
If, for example, your divide the frame vertically (see the white division lines) by placing a strong horizontal line at either the lower or upper one-third division, you create a pleasing sense of proportions between the upper and lower halves of the scene. In the sunset here, for instance, by placing the horizon line approximately one third of the way down from the top of the frame, I've created a 2:1 ratio between the lower and upper halves of the scene and the brain perceives this as having stability and balance. The idea of thirds is further reinforced in this shot by placing the small clump of dark land in the foreground at approximately the lower one-third line.
You can also use the intersection of thirds lines as a helpful guide in placing important subject elements, even if there is no particularly strong line present--placing an object in a still life where thirds intersect, for example. In this shot I've put the heavy clump of land at the intersection of the lower one-third and left-vertical third lines and that sets up a nice feeling of balance with the open area of water to its right. In fact, the lower third line seems to run right through the center of the clump of earth.
Would the image fail if I moved either the horizon or that clump of ground to a slightly different position in the frame? No, of course not. Dividing the frame is as much about personal taste and instinct as it is about rigid divisions of the frame. But artists have been using this simple technique for centuries and I think you'll find that when you're searching for just the right balance of objects, spaces and lines, the suggestion (not the rule!) of thirds can be very useful.