"For around 2000 years single and double curved shells structures, such as barrel and vault s, have been used to cover large spans in buildings. Until the twentieth century these were generally constructed either from masonry or some form of unreinforced concrete, materials strong in compression but relatively weak in tension. Well known examples such as the Pantheon... ... Santa Maria del Fiore... and St. Peter's Basilica... have a span to thickness ratio of less than 50 to 1, which is relatively thicker than a... typical hen's egg. ...[T]he stone vaulting of... medieval Gothic cathedrals... demonstrate the mason's art in the construction of... complex masonry shells. With the advent of reinforced concrete... strong in both compression and tension, it became possible... to construct thin shells with much higher span to thickness ratios... commonly... in the region of 500 to 1."