Forms have length and width but they also have depth and three dimensions.

Space will often define or dominate the form

You can actually experience a form by walking around it, such as a sculpture or architecture

Natural and man-made objects are created by combining basic geometric forms

Solid forms resist the penetration of space also called a closed form.  

If space is allowed to invade the form it is referred to as a penetrated or open form

  Pelagos 1946, Part painted wood and strings object: 368 x 387 x 330 mm