The Rule of Thirds

This is a common rule that is applied by many photographers. It works by dividing the image into a grid using two vertical and two horizontal lines, which split the picture into thirds both horizontally and vertically. The ideal location of the subject should be where the lines intersect.
Rather than placing the horizon in the middle, you should place it on one of the horizontal lines. Placing the ubect to one side or the other, on the vertical lines, gives the viewer more oportunity to look into other areas of the image.
As with all of these rules however, they are there to be broken. The trick is to know when.