Now to the part everyone photographs. The lower section of the facade was started in 1350, in the medieval spirit. Above it, the great Renaissance architect Leon Battista Alberti designed the upper part, dated to the years 1456 to 1470. He worked in inlaid green marble from Prato and white marble, creating a pattern of crisp squares and circles. Alberti's challenge was delicate. The church below him was already medieval, so he could not simply start fresh. He chose to harmonize the new with the old, applying classically inspired proportions to make one unified whole. The two great S-shaped scrolls on either side of the upper facade have no precedent in ancient classical architecture, so they are his own invention, linking the tall nave to the lower side aisles. Alberti was also a writer. His book On the Art of Building, inspired by the Roman architect Vitruvius, became the first major architectural treatise and a guide to Renaissance design. The facade has a final footnote: its right side, the last part left incomplete, was added only in 1920.