KEY, or Key-stone, of an Arch or Vault, is the last stone placed on the top thereof, which, being wider and fuller at the top than bottom, wedges, as it were, and binds together all the rest. The key is different in the different orders. In the Tuscan and Doric it is only a plain stone projecting; in the Ionic it is cut and waved, somewhat after the manner of consoles; in the Corinthian and Composite it is a console enriched with sculpture, foliage, and other ornaments.
Key is also used for ecclesiastical jurisdiction, particularly for the power of excommunicating and absolving. The Romanists say the pope has the power of the keys, and can open and shut paradise as he pleases, grounding their opinion on that expression of Jesus Christ to St Peter, "I will give thee the keys of the kingdom of heaven." In St Gregory we read that it was the custom for the popes to send a golden key to princes, in which they enclosed a little of the filings of St Peter's chains, kept with a world of devotion at Rome; and that these keys were worn in the bosom, as being supposed to contain some wonderful virtues.