The rule of thirds applies to the process of composing photographs and helps you to shoot visually appealing and balanced images. How? Simply by dividing the image you’re going to shoot into thirds, horizontally and vertically, in order you get nine equal parts. By doing so you will have a grid. The points where horizontal and vertical lines intersect are called also points of interest or anchor points.
Composing your photo in order that the subject of your photo stands at or near one of those anchor points, can help you to achieve a strong composition. You will get an image with more tension, energy and interest in the composition than simply centering the subject would. The rule of thirds tells you that if you locate the main subject of your image in the top, bottom, left, or right sections of the grid then you will have the right shot.
The photograph above demonstrates the application of the rule of thirds. The horizon sits at the horizontal line dividing the lower third of the photo from the upper two-thirds. The tree sits at the intersection of two lines, sometimes called sweet spot. By aligning the subject of your photo along these lines or their intersections, you get an image with more tension, energy and interest in the composition than simply centering the subject would.
The main reason for observing the rule of thirds is to discourage placement of the subject at the center, or prevent a horizon from appearing to divide the picture in half. As you look through the viewfinder of your camera at a scene, place the subject at one of the imaginary grid intersection points in order the image gets an overall dynamic balance.
This rule is so popular in portrait and landscape photography that all digital cameras come with a rule of thirds grid overlay that you can enable while you’re shooting.