The church interior is both extraordinary and complex. The three principal parts can be identified vertically as the lower order at ground level, the transition zone of the pendentives and the oval coffered dome with its oval lantern.
In the lower part of the church, the main altar is on the same longitudinal axis as the door and there are two altars on the cross axis. Between these, and arranged in groups of four, sixteen columns carry a broad and continuous entablature. The arrangement seems to refer to a cross plan but all the altars are visible as the two central columns in each arrangement of four are placed on the oblique with respect to the axial ordering of the space. This creates an undulating movement effect which is enhanced by the variation in treatment of the bays between the columns with niches, mouldings, and doors. Architectural historians have described how the bay structure of this lower order can have different rhythmic readings[4] and the underlying geometric rationale for this complex ground plan, as well as discussing the symbolism of the church and the distinctive architectural drawings of Borromini.[