The "rule of thirds" is a simplified version of the golden mean. The rule of thirds is a basic technique that photographers learn to frame their shots. Subjects placed exactly in the middle can often make for an uninteresting photo. The golden mean would be wonderful to apply when taking snaps, but obviously this is not practical. But a viewfinder can be divided by lines — real or just imagined — so that you have four intersecting lines or crossing points and 9 rectangles that resemble a tic-tac-toe board. These four crossing points (also called power points, if you can believe it) are areas you might place your main subject, rather than in the center.
There is a belief, and some research to back it up, that we are hardwired to naturally be drawn to images that have proportions approaching the golden mean, just as we are often drawn to many things in the natural environment — or even to a particular "good-looking face" — with golden-mean like proportions.
The idea of objects and visuals having proportions close to the golden section is fascinating stuff for architects, mathematicians, designers and artists. And the rule of thirds is a very basic and useful technique for photographers in many cases. But what does it mean for us? What happens if we take, say, an 800x600 PowerPoint or Keynote slide and divide it up? Dividing our "canvas" into thirds is an easier way to at least approach golden mean proportions. The rule of thirds is not really a "rule" at all, but rather a guideline. But you will find that you can apply this guideline even to PowerPoint or Keynote visuals to give them a more symmetrical and professional look. Here are some sample slides below.
Above, the statistics for this particular point are placed directly on two of the four "power point" crossing sections. The face of the woman is in the right third and near a crossing section.
Out of curiosity, I placed a golden mean rectangle over the image that comprises the homepage of my website. I did not use a grid to crop the photograph or for placing the text. I went with what "felt" right. Interestingly, when I place the golden rectangle over the image, I found that the proportion of the webpage image is close to the golden rectangle, but more interesting is that the placement of my name over the image on the right is in exact golden mean proportion horizontally.
Go back and take a look at one of your recent PowerPoint or Keynote presentation slide decks. In the slide master, use the grids to divide your slides into thirds ("ctrl/command G" from your keyboard will turn the grids on/off). How can your arrange your slide content to be in better proportion? Does it make a difference?
Here is a very good, basic (and visual) explanation of the golden mean and the rule of thirds.