For beginners out there, quoting Wikipedia :
The rule of thirds is a compositional rule of thumb in visual arts such as painting, photography and design. The rule states that an image should be imagined as 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.
You can have this grid to switch on and off in Photoshop and Elements and I suppose any other editing software out there where you can define your grids.
In Photoshop you go to
Preferences, move down and choose "
Guides, Grids & Slices... " Set for Gridline at
33.3 and change the pixels/cm to
Percentage. Then change subdivisions to
1.
If you are using a Mac like me then pressing the
command and ' keys lays the grid over your picture to check thirds. Repeating the key stroke switches it off. I'm not too sure of the keystroke sequence with Windows but I think it is similar.
This link takes you to the Wikipedia entry where there is an example to illustrate the rule with a grid placed over it.
LINK
Hope I'm not teaching granny to suck eggs but I find it useful.