The period from 1932-34 proved to be one of Gerrit Rietveld's most productive times. During these years he made the first prototypes for his Zig-Zag Chair. The first prototype is made out of a frame of iron strips covered in a single sheet of fibreboard. Structurally, there were problems. The fibreboard could not sustain stress at the sharp edges and at the corners of the frame. In Rietveld's second prototype, he uses a steel-strip frame which is screwed into pieces of plywood. Both the plywood and steel strips are vital elements for the structure and design. Later, Rietveld dispenses with the steel and screws together the sections of plywood to have a chair entirely composed of wood. The Zig-Zag Chair was Rietveld's best selling design.