Calling the "Rule of Thirds" a rule is perhaps overstating its use, as with anything in the creative world it is a guide and not something to be applied with mathematical rigidity. The philosophy is not to place the main area of interest in the centre of the image. Instead, an imaginary grid divides the image into horizontal and vertical thirds. Where the lines intersect is a point of visual power.
The first example is a famous artwork, The Fighting Téméraire tugged to her last Berth to be broken by J. M. W. Turner, painted in 1838 and now hanging in the National Gallery in London. To make it clearer, a Rule of Thirds grid was superimposed on the image of the painting, using the Gimp.
One of the power points is located near the main subject, the ships. The suggested painting technique is to draw in light pencil the Rule of Thirds grid on the canvas. Clearly, Turner did not feel the need to apply the rule exactly. However, Turner did follow the principle of offsetting important elements away from the center.
The intersection points are not the only influence of the Rule of Thirds. The grid lines are useful for determining horizontal and vertical balance. One primary consideration is where to place the horizon. Here, Turner chose to align the horizon with the lower horizontal grid line.
This accommodates the tall subject, although the bare masts lack visual weight, making the image bottom-heavy. To counteract the excessive visual weight in the bottom left corner, Turner added the colorful sunset reflected in the clouds and on the water in the right side of the image.
Another consideration of the Rule of Thirds is that motion or directional indicators should normally be towards the center of the image. The subject of Turner’s painting, the warship, is on the left it, and appears to be travelling to the right of the image. There is a natural inclination by most viewers to follow that direction to see where the warship is going. In this case, it is the reflected sunset in the clouds and on the water.
The Rule of Thirds applies to portraiture as well as landscape images. The image of the Kawasaki girls from Laguna Seca lines up with the rule’s grid. The most important points in a portrait are the eyes. Even though they are all wearing sunglasses, the two top points of power coincide with the eyes of the two girls bookending the group. This provides horizontal and vertical and balance and of course, both outer girls are slightly facing in towards the center of the image.