Senna,,,The rule of thirds is a compositional rule of thumb in photography and other visual arts such as painting. The rule states that an image can be divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines. The four points formed by the intersections of these lines can be used to align features in the photograph. Proponents of this technique claim that aligning a photograph with these points creates more tension, energy and interest in the photo than simply centering the feature would.
Thanks Bobbiel.
I had a look at my last, say 20, pictures, with your explanation in mind. And I noticed that in half of them I used this rule of thirds, apparently just intuitively.