The chapel of the Santissimo Sacramento is part of the complex of the Cathedral of San Martino in Lucca, built in the first half of the sixteenth century by the architects Nicolao and Vincenzo Civitali; a recent stylistic reading attributes the idea to Baccio da Montelupo, at that time present and active in Lucca.
The chapel has been inserted into the right transept that previously described a Latin cross basilica, characterized by short arms secondary to the main nave and side aisles. For this purpose, the two walls that defined the presbytery and the right arm were opened, creating two large arched bays in the corners, framing two identical internal facades.
The exterior followed the pre-existing structural lines, marking the junctions with simple, severe corner piers. Looking at the exterior of the chapel, one can clearly see the simplicity of the achitectonic lines. It’s therefore fairly easy to identify the basic geometry and proportional system.
As previously mentioned, the juxtaposition of the previous structure (XIV century) and the chapel has been addressed by creating large, smooth piers that are only horizontally punctuated by frames, which are widely spaced. The piers and the frames identify a double square, which is thus the basic geometry of the chapel. The first square (best seen by looking at the east elevation) is defined by the ground line, the two corner piers and the double frame; the second square is immediately above. Tracing a circle inscribed in the second square we find the arched frame that characterizes the upper part of the chapel.
The same proportional scansion can also be performed on the inside, where the large intermediate frame delineates the two squares. In this case, however, the spaces defined by the squares are not left "empty," but are further broken-down using arches, pedestals, columns, frames and tympana, in an architectural lexicon of a now-mature Renaissance.
Watch the slide show to see the underlying ratios and systems that Federico used to generate his accurate drawing of the facade
See the system of the interior articulation overlaid on a two-point perspective here