I took this photo in an elevator on a recent trip to Italy. Notice the buttons are labelled negative one and zero. In the USA, I’ve never seen an elevator with floor “0”, never mind floor “-1”.
This elevator is in a train station, and goes between ground level and an underground tunnel that allows people to cross beneath the train tracks.
In many cultures, including Italian, the “1st floor” is the one above ground level — the one we would number the “2nd floor” in USA English. So it’s logical that ground level could be considered floor number zero. It’s as though the Italians use the C programming language convention of numbering arrays starting with 0, while in the USA, we use the FORTRAN convention of starting with 1.
By logical extension of the number line, the Italian convention for the first level below ground is negative one.
In the USA, such an elevator might have floors labelled “G” and “B”. I like the mathematically logical Italian way. Plus, it gave me the opportunity to give a quick math lesson to my son.