What is helicoidal, you wonder. Me too. Every time anyone would talk about this elegant, snug, and lovely Borromini staircase in the guidebooks or on the web they'd toss in helicoidal as soon as they could. And let's face it, helicoidal is a very nice sounding word. I find it to be extremely well suited to this staircase.
So naturally we tried to track down someone at the museum who could tell us how to say "helicoidal" in Italian. We finally found a friendly, sort of bilingual security man. He had no idea what we were talking about, but finally he took out his smart phone and looked up a translation. The Italian word is apparently "elicoidale". When I asked the guard what elicoidale meant, he said he did not know. No one does.