However, the principal and most important architectural feature in the church is the elliptical dome. Within it are geometric designs (crosses, octagons, hexagons) and a lantern with the symbol of the Holy Spirit. Most of the light comes from the dome, which has no drum; further light, though not as much, comes through the window above the entrance. A ring of large, stiffly carved leaves stands out in silhouette against the complicated coffered pattern of the dome, like some strange crowning balustrade. Borromini set about in an ingenious way to be able to apply the dome onto the irregularly shaped interior.