This etching bears a unique title: “Cross-section of the Interior of the Basilica San Paolo fuori delle Mura.” Typically, the etchings in the Views of Rome are called views or “vedute,” as the name of the series suggests, but by using the word “cross-section” or “spaccato” in the title, Piranesi indicates to readers that the function and meaning of this image is different from the others in the volume. “Cross-section” is a technical term Piranesi employs frequently in the Antichità Romane among his other archeological and architectural publications. The term indicates a particular type of perspectival projection that is often planar and abstract in style, rendered to scale, and replete with measurements, annotations, and objective details in the key, as in the image below. 
Similar to the image above, which shows the cross-section of an ancient tomb, the upper left corner features a “unique device of an architectural section through the nave wall” (Wilton-Ely, 31), which reveals the precise structural and spatial dimensions of the aisles. Also similar is the way that Piranesi uses different types of etching lines to indicate the texture of the wood, marble, stone, and brick. The upper level of the aisle wall, as seen in the detail below, appears as though it has been sliced in half and is particularly flat through the use of sparsely punctuated lines.
This visual emphasis reinforces the highly technical language in the text about the distribution of weight to sustain the massive interior composed of five naves and no less than eighty marble columns. Furthermore, as art historian John Wilton-Ely notes, the interior is rendered to scale in precise mathematical perspective (Wilton-Ely, 29-31). While scale and perspective are often exaggerated in the Views of Rome, here accuracy prevails. Such attention to scale is also a typical feature of the Antichità Romane. So too is the level of objective detail in the text. Indeed, Piranesi includes a lengthy excursus on the history of the church, as well as the methods and materials used in its construction: for example, the use of Greek marble in various colors and the reuse of marble fragments to create the church floor. While this image shares many features with the archeological print, the variety of figures in the foreground, playful effects of light and shadow, and visual detail also show the vibrance of eighteenth-century life and tourism characteristic of the Views of Rome.  


To see this image in Vedute di Roma, vol 16 of Piranesi's Opere, click here.