Artist 
Writer Rabatment of the rectangle
If you take the short side of the rectangle and make a square out of it, the side of the square is a good one to include in your composition. In other words, it makes a perfect square within the rectangle of the painting support.
This is called the rabatment of the rectangle. You can put objects on the rabatment line to give a sense of structure and harmony.
See the Composition Building Block for more information.