What do the Pantheon & St. Ivo Have in Common?

I've just discovered that Borromini reprised the Pantheon's ancient stepped-ring dome in his c.1650 charming Baroque design of St. Ivo.

It's not something one readily notices from street level.

But, when photographing the dome of the Pantheon from the roof of a building on the Piazza della Rotonda, I captured this image of Borromini's delightful, fanciful corkscrew cupola atop St. Ivo in the near distance.

Large-as-life ... stepped rings on the dome beneath the bell tower!