A basic design principle, the rule of thirds simply involves imagining lines dividing an image into thirds, both horizontally and vertically. Subjects placed at any of the four intersections of these lines make a more compelling image than if placed in the center or at a far corner.

In this photo, the subject falls within the top and right thirds of the frame, leaving "blank" space below her. The empty space adds drama and leads the eye, resulting in a more effective shot.