When pilgrims pass under the arch and into the nave (the long interior hall), after their eyes adjusted to the purposefully dim, mystical light, they would look up and see a second, much more massive barrel vault, the largest constructed since ancient Rome.
Then, on both sides of the nave they would find three chapels with lower barrel vaults. Surprisingly, there are no side aisles or rows of columns, as at the old St. Peter’s in Rome or other early churches like Santa Sabina.
Sant’Andrea’s huge central space and buttressing side chapels strongly resemble the layout of the ancient Basilica of Maxentius and Constantine in the Roman forum (below). The basilica plan is perfectly suited to large churches since it could accommodate massive crowds. But unlike earlier basilica-plan churches, Sant’Andrea's plan seems to return more strictly to the ancient forms.