Most key rings carry one key that nobody can place. It fits nothing anyone can remember, not the front door, the car, the shed or the bike lock, and it has ridden along for years, through a move or two. It stays because throwing it away is the only test that would settle the matter, and the answer would come at the worst moment, outside some door, late, in the rain. So it keeps its place on a technicality. Every other key on the ring is carried because it works; that one is carried on faith.