You haven't been around much, then. The seven-segment display inside the elevator in my former dormitory labelled the ground floor as '0', and the one below as

You haven't been around much, then. The seven-segment display inside the elevator in my former dormitory labelled the ground floor as '0', and the one below as '-1'. There was also a voice announcer, but I don't remember what it said on floor -1; I didn't go there often.