Peter’s Basilica. Lines of arched frame establish rows of posts or columns. Arched construction is characterized by buttresses which are often quite deep, to take arch thrusts; walls which are minimized, both in length; large openings; enormous windows and accent on verticality (Gorman 88). The arched frame determines all major architectural lines. Buttresses and flying buttresses accent the structural pattern.
The architects of the Renaissance that designed St. Peter’s Basilica sought for a dome that was more like the dome of the Pantheon. The Pantheon dome is merely a hemispherical vault, built in rings of stone or brick, with the joints between the rings radiating from the center, and the joints of each ring also necessarily radiating as well (Dutemple 81). Thus, the completion of each ring makes a self-supporting unit, because, owing to the wedge shape of the elements of which it is combined, no one of them can collapse inward; it is this quality which enabled the Romans to build domes with its great “eye” or circular opening in the center. For the architects of St. Peter’s Basilica it was necessary at the same time to have a high dome to give external effect because of the length of their churches. Consequently a dome of two or even three shells was developed, in which the interior dome was proportioned with sole reference to interior effect, and the exterior dome with sole reference to exterior effect (Moffett 143). Between these two shells there was sometimes a third, built to carry the weight of the “lantern,” the small, many-windowed cupola which took the place of the “eye” of the Roman dome. Such domes may be seen not only in St. Peter’s in Rome, but, also, in St. Paul’s in London, in the Panthéon and Les Invalides in Paris,