In photographic composition, the Rule of Thirds is a good guideline which you can use to help you create photographs with greater visual impact. It is a compositional device which is also used by painters and other visual artists
All you have to do is to imagine that the camera's viewfinder is divided into thirds, both horizontally and vertically.
Position your subject at the intersection of those lines and it will be placed one third of the way from the sides, both top and bottom. This is illustrated in the image above of an old abandoned windmill in France.
Some cameras have a means whereby you can actually see a grid in your viewfinder which will help you position your subject. If not, you can easily find it with a bit of practice.
Another way to apply the Rule of Thirds is by cropping the picture afterwards. The shot below was the original shot I took of the windmill but with a wider view of the scene. Unfortunately the composition is less dynamic so cropping was in order.
Here is the shot again (below) after being cropped but without the grid lines. Clearly, a better composition, tighter and more pleasing. Irrelevant detail which distracts the viewer has been eliminated.
Note that when cropping it helps to keep the dimensions in the same ratio as the original otherwise you end up with a series of photographs of odd sizes which tends to look a bit amateurish.
The last shot below is of a second windmill which I took after moving around the field a bit. It also has the main subject placed using the Rule of Thirds. I have kept the composition very simple too which strengthens its appeal. The contre jour lighting from the sunset behind the windmill was what particularly attracted me to make this shot but I had to tweak it a little in Photoshop to bring out some detail from the shadows. Being up much closer also adds to the drama.
According to Henri Cartier-Bresson (1908-2004): "...it is seldom indeed that a composition which was poor when the picture was taken can be improved by reshaping it in the dark room." In spite of what this famous master tells us, cropping a photograph afterwards can be useful.
Use cropping to get a feel for what works compositionally but with practise you won't need to worry about the so-called 'Rule' and will be able to instinctively position your subject and your photographic composition will be the better for it .