- The photographer's rule says: place important objects at 1/3 lines, ideally where two of them intersect.
- Everyone else's says: divide the image up into regions on 1/3 lines and place things within the regions.
In order to underline this, I have selected at random some iconic photographs. Which, of course means, not random at all. These are simply the first few that occurred to me, so who really knows what my subconscious dredged up. Anyways. These all have a helpful grid of 1/3 lines placed on them, just as all those terrible "pep up your snaps" web pages have to show you where to stick the boat, or the flower, or the little girl's eyes, to make the photograph look awesome and professional.
Let's see what actual professionals, producing actual superb images do. You may not recognize all of these, but each of these images has been hung in museums, most of them (if not all) multiple times:
How often are things of importance placed on 1/3 lines? How often are things of important placed at the intersections of 1/3 lines? How often is the image divided into thirds vertically, with important things placed within the three resulting horizontal bands? How often is the image divided into thirds horizontally, with important things placed within the three resulting vertical bands?
Think up some other photographs which are iconic enough that you remember them. Use google to find a copy, and place your own grid of thirds on it.
Don't listen to those idiots with their idiotic canned advice about composition. Go look at some good photographs instead.