Rule of Thirds
The rule of thirds is a compositional tool that makes use of the belief that the most interesting compositions are those in which the primary element is off centre.
It applies to the process of composing paintings and photographs so that an image should be imagined as being divided into nine equal parts, by two equally spaced horizontal lines and two equally spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections.
Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject would.
The rule of thirds was documented by John Thomas Smith in 1797. In his book, Remarks on Rural Scenery, Smith quotes a 1783 work by Sir Joshua Reynolds, in which Reynolds discusses, in unquantified terms, the balance of dark and light in a painting.
Smith then continues with an expansion on the idea, naming it the "Rule of thirds":
"Analogous to this "Rule of thirds", (if I may be allowed so to call it) I have presumed to think that, in connecting or in breaking the various lines of a picture, it would likewise be a good rule to do it, in general, by a similar scheme of proportion; for example, in a design of landscape, to determine the sky at about two-thirds ; or else at about one-third, so that the material objects might occupy the other two : Again, two thirds of one element, (as of water) to one third of another element (as of land); and then both together to make but one third of the picture, of which the two other thirds should go for the sky and aerial perspectives. This rule would likewise apply in breaking a length of wall, or any other too great continuation of line that it may be found necessary to break by crossing or hiding it with some other object : In short, in applying this invention, generally speaking, or to any other case, whether of light, shade, form, or color, I have found and flexibility".
This youtube video provides a simple example