These spiral stairs, in the Pio-Clementino Museum in Vatican City, take the form of a double helix — there are two heads and two feet, so that one party can descend while another ascends and the two will never meet.

Designed in 1932 by Giuseppe Momo, the stairs are modeled on a much earlier stair by Donato Bramante, which occupies a square tower in the Belvedere palace of Pope Innocent VIII. Like the museum stair, it allows traffic to travel in both directions without impediment.