The rise of baroque architecture prompted Borromini to bring his sculpture background to life by creating unexpected combinations of curves and rectangular forms in his work.
Many baroque architects during the seventeenth century focused their design basis to fall in line with proportions of the human body.
Borromini was non-classical in the sense that he based his designs on geometric figures.

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.
One altar is dedicated to Saint Michael de Sanctis, the other dedicated to Saint John Baptist of the Conception.
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 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.

The pendentives are part of the transition area where the undulating almost cross-like form of the lower order is reconciled with the oval opening to the dome.
The arches which spring from the diagonally placed columns of the lower wall order to frame the altars and entrance, rise to meet the oval entablature and so define the space of the pendentives in which roundels are set.

The oval entablature to the dome has a 'crown' of foliage and frames a view of deep set interlocking coffering of octagons, crosses and hexagons which diminish in size the higher they rise.
Light floods in from windows in the lower dome that are hidden by the oval opening and from windows in the side of the lantern.
In a hierarchical structuring of light, the illuminated lantern with its symbol of the Holy Trinity is the most brightly lit, the coffering of the dome is thrown into sharp and deep relief and light gradually filters downwards to the darker lower body of the church.

Flanking the apse of the main altar is a pair of identical doorways.
The right door leads to the convent through which the crypts below may be accessed.
 The door on the left leads to an external chapel known as Capella Barberini which contains a shrine to blessed Elisabeth Canori Mora.
The rise of baroque architecture prompted Borromini to bring his sculpture background to life by creating unexpected combinations of curves and rectangular forms in his work. Many baroque architects during the seventeenth century focused their design basis to fall in line with proportions of the human body. Borromini was non-classical in the sense that he based his designs on geometric figures. 
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. One altar is dedicated to Saint Michael de Sanctis, the other dedicated to Saint John Baptist of the Conception. 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 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.
The pendentives are part of the transition area where the undulating almost cross-like form of the lower order is reconciled with the oval opening to the dome. The arches which spring from the diagonally placed columns of the lower wall order to frame the altars and entrance, rise to meet the oval entablature and so define the space of the pendentives in which roundels are set.
The oval entablature to the dome has a 'crown' of foliage and frames a view of deep set interlocking coffering of octagons, crosses and hexagons which diminish in size the higher they rise. Light floods in from windows in the lower dome that are hidden by the oval opening and from windows in the side of the lantern. In a hierarchical structuring of light, the illuminated lantern with its symbol of the Holy Trinity is the most brightly lit, the coffering of the dome is thrown into sharp and deep relief and light gradually filters downwards to the darker lower body of the church.
Flanking the apse of the main altar is a pair of identical doorways. The right door leads to the convent through which the crypts below may be accessed.  The door on the left leads to an external chapel known as Capella Barberini which contains a shrine to blessed Elisabeth Canori Mora.
San Carlo alle Quattro Fontane