Rule of Thirds

The Rule of Thirds is a basic design principal intended to help with the composition of an image. The idea is that you draw two equally-spaced lines both horizontally and vertically across your canvas, effectively cutting it into thirds in both directions. Where these lines intersect are the points that would be effective focal points for your image.

Here is one example of this, from my own archives.

If you imagine the lines, you'll notice that the medallion at the top of the marker comes very close to the top left intersection. This makes for a dynamic composition.