Naum Gabo (1890-1977) was a pioneer of the Russian Constructivist art movement. His work utilized what is known as "stereometric construction," by which form is achieved through the description of space rather than mass. In explaining the method he once stated, "We take four planes and we construct with them the same volume as four tons of mass." He felt his Head No. 2 sculpture created in 1916 (pictured below), was his first masterpiece and I would have to agree; the piece is mesmerizing.