The rule of thirds
The rule of thirds is not a true rule, is more a “rule of thumb” or guideline which applies not only to composing photographs but also in visual images in general (such as designs, films, or paintings).
It says that an image should be imaginarily divided into nine equal parts by two equally spaced horizontal lines and two equally spaced vertical lines, and that the important compositional elements should be placed along these lines or their intersections. The theory is that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject. See for yourself some examples below.
Do not confuse the imaginary lines with the dots that may appear on your viewfinder while shooting. Those are the AF (AutoFocus) points, they have nothing to do with the rule of thirds.
But – again – this is one of those rules that I break all the time. But it’s always on the back of my mind while I’m shooting (or editing). And don’t ask me why but it usually works. Unless you want to create something more disruptive, like for instance, filling an image with negative space and having the focus and the subject on a corner or to a side of the image.