KEY, or Key-stone, of an Arch or Vault, is the last stone placed a-top thereof; which being wider and fuller at the top than bottom, wedges, as it were, and binds all the rest. The key is different in the different orders: in the Tuscan and Doric it is a plain stone only 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, &c.
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 Peter, "I will give thee the keys of the kingdom of heaven." In St Gregory we read that it was the custom heretofore for the popes to send a golden key to princes, wherein they inclosed 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.
KEY is also used for an index or explanation of a cipher. See CIPHER.
KEYS of an Organ, Harpsichord, &c. those little pieces in the fore part of those instruments, by means whereof the jacks play so as to strike the strings. These are in number 28 or 29. In large organs there are several sets of the keys, some to play the secondary organ, some for the main body, some for the trumpet, and some for the echoing trumpet, &c.: in some there are but a part that play, and the rest are only for ornament. There are 20 slits in the large keys which make half notes. See the article ORGAN, &c.