There are a number of compositional structures that artists use. The cantilever structure is when any large formation enters the art work from a side and ends within the work. Normally the cantilever connects with other positive shapes/formations in the art but it can stand alone. Sometimes cantilevers meet with similar ones entering from the other side. The heavy visual weight of a cantilever usually must be balanced by another positive shape. Artists such as Cezanne and Matisse frequently used the cantilever structure in their paintings. A chapter on compositional structures is in the excellent book by Nathan Goldstein, Design and Composition.