1. The stone at the summit of an arch, which, being the last put in, is looked upon as locking the whole together.
a. 1637. B. Jonson, Underwoods, Misc. Poems xxx. To Sir E. Sackville. ’Tis the last key-stone That makes the arch.
1703. Moxon, Mech. Exerc., 279. If you will add a Keystone … to the Arch … let the breadth of the upper part of the Keystone be the height of the Arch.
1790. Burns, Tam o’ Shanter, 206. Now, do thy speedy utmost, Meg, And win the key-stane of the brig.
1851. Ruskin, Stones Ven., I. x. § 4. One voussoir is as much a keystone as another; only people usually call the stone which is last put in the keystone; and that one happens generally to be at the top or middle of the arch.
1858. Mrs. Oliphant, Laird of Norlaw, III. 272. The narrow door, with some forgotten noble’s sculptured shield upon its keystone.
b. fig. Something occupying a position compared to that of a keystone in an arch.
1641. J. Jackson, True Evang. T., III. 182. Christian Society is … like stones in an arch, one holds and fastens another, Christ himselfe being the key-stone.
1790. Burns, Tam o’ Shanter, 69. That hour, o’ night’s black arch the key-stane.
1839. Bailey, Festus, i. (1852), 10. The sun, centre and sire of light, The keystone of the world-built arch of Heaven.
1866. Howells, Venet. Life (1883), I. xiii. 245. At the other end of the saloon sat one of the fathers, the plump key-stone of an arch of comfortable young students.
c. esp. The central principle of a system, course of action, etc., upon which all the rest depends.
1817. Coleridge, Biog. Lit., 96. Religion, as both the corner stone, and the key-stone of morality.
1832. Lewis, Use & Ab. Pol. Terms, xvii. 163. The keystone on which all government must ultimately rest.
1849. Macaulay, Hist. Eng., vii. II. 166. The tenet of predestination was the keystone of his religion.
1876. Rogers, Pol. Econ., ix. (ed. 3), 108. The principle of unlimited liability is the keystone of the system.
2. A bond-stone.
1823. P. Nicholson, Pract. Build., 339. Key-Stones, a term frequently used for bond-stones.
3. In chromolithography: see quot.
1875. Ure’s Dict. Arts, III. 135. A drawing of the subject, in outline,… is made … when transferred to a stone, this drawing is called the keystone, and it serves as a guide to all the others, for it must be transferred to as many different stones as there are colours in the subject.
1889. G. W. Clement Smith, in Pall Mall Gaz., 23 Jan., 3/1. These [‘offsets’] are tracings of those portions of matter in the keystone which are to go in each colour, an offset for the red, one for the blue, and so on.
4. A block of cast-iron used to fill up certain spaces in a Scotch lead-smelting furnace.
1857. Tomlinson, in Encycl. Brit. (ed. 8), XIII. 300/1. (Lead) The space at each end of the fore-stone is closed by a cube of cast-iron called a key-stone: two similar stones fill up the space between the fore-stone and the back part of the furnace.
5. attrib. and Comb., as keystone-mask; keystone-mill, a kind of mill used for grinding tanning materials; Keystone State, U.S., popular appellation of Pennsylvania, as being the seventh or central one of the original thirteen states.
1881. Spon’s Dict. Indust. Arts, 1227. The well known American ‘keystone’ mill.
1890. Daily News, 26 Nov., 7/3. The quite famous sculptured keystone-masks on the east and west sides of the central arch of Henley Bridge.
Hence Keystoned a., having a keystone.
1887. T. Hardy, Woodlanders, I. iv. 52. Under that keystoned doorway.
