Did you know, that at the turn of the 20th century, it was fashionable for a woman to wear a necklace with several keys? Apparently, the more keys that a woman wore around her neck, the more rooms she had in her house. That translated into wealth, as that meant that she was married to a man with money. At that time, keys were a status symbol.
But perhaps the most important key could be the one held by St. Peter!