Interior View of St. Peter's at Rome
"The decoration of the interiors of the buildings of the Renaissance is also copied from ancient Roman architecture. The rooms are either vaulted or have flat ceilings, but in both cases are adorned with paintings after the manner of those discovered in the Baths of Titus, or by panel-work, that is, sunken coffers with a regularly distributed enrichment [shown here]. These panels are themselves often adorned with historical or allegorical paintings, or with arabesques. Ornamented panels were employed in large palaces for horizontal ceilings, as also in churches, though in the latter case they were more often applied to cupola vaulting, as notable in St. Peter's."
A. Rosengarten, W. Collett-Sandars A Handbook of Architectural Styles (New York, NY: Charles Scribner's Sons, 1895)