When a form intercepts a parcel of direct light, it projects or casts a shadow onto whatever lies behind it.
The resulting cast shadow can be a striking design element, as it was for Frank Brangwyn in one of his famous bridge paintings.
Sometimes forms outside the composition cast shadows onto the subject. The movement of the morning sun shifted the shadow fairly quickly over the Flatiron building.
Samuel Prout effectively used the cast shadow in this watercolor of the Palazzo Contarini in Venice.
The edge of the shadow shape follows the bold relief of the building. It also sets up opportunities for tonal design. Some figures are seen in light against shadow shapes, while other figures are in relative darkness against a bright background.