If you take a rectangle, any rectangle will do, and draw a square within it. Usually by measuring one side and then finding that measurement across the rectangle, like so....
You now have an attractive mathematical armature upon which to build a design. As I mentioned before, the viewer finds hidden geometry pleasing in a painting. I think that is because they are vaguely aware that there is sense or order there, even if they don't know exactly why. Below are a couple of paintings that use this idea.
This Payne uses the square as an opening or window into his distance. Below is a variation on the idea by Eastman Johnson. The square is a little smaller but the idea is the same.
This square functions like a stage on which Johnson's actors pose. Can you find the square in this landscape by Enneking?
Above is a Leon Gerome from the artrenewal.com showing the same idea again. Napoleon and all of his roadies are confined within the square. The guy in the red dress defines the outer edge of the square.