What feature ties the two sections of the Piazza of St. Peter's together?

The feature that effectively ties the two sections of the Piazza of St. Peter's together is the Ionic entablature. This architectural element creates a visual continuity across the space by incorporating classical design principles, which were heavily influenced by ancient Roman architecture. The use of the Ionic order in the entablature emphasizes elegance and harmony, serving as a unifying element in the overall composition of the piazza.

The Ionic entablature not only adds aesthetic value but also helps to guide the visitor's eye through the space, connecting the two sections seamlessly. This reflects the Renaissance ideals of symmetry and proportion, as well as their reverence for classical antiquity.

Other features mentioned, such as a classical dome or rectangular courtyard, do play important roles in the design of the piazza but do not serve to unite the two sections as effectively as the entablature does. The dome, while iconic, is separate from the piazza's layout, and the courtyard's shape, while significant, does not enhance the cohesion between sections in the same way that the entablature’s design does.