The rectangle represents the confinews of the world to a painter. calling attention to the rectangle is the kind of thing the Modernist Abstract painters worked with, Mondrian  for a good example.

 

Often finding ways to direct the eyes around the picture to bring the eye into the picture is what most classical composition is about.  

 

This painting is a classical fruit in bowl painting with a twist of the rectangle in the composition relating to the canvas edges.  The round fruits are wuite flat and the rectangle is deep and modeled.