Usage examples "keystone":
When it was determined to convene the Continental Congress Mr. Lewis was unanimously elected a member by the delegates convened for that purpose on the 22d day of April 1775. He immediately repaired to the Keystone
city and entered upon the important duties assigned him. - "Sages and Heroes of the American Revolution",
L. Carroll Judson.
" It may seem strange," Mrs. Shelley writes, " that any one should, in the sincerity of his heart, believe that no vice could exist with perfect freedom, but my father did; it was the very basis of his system, the very keystone
of the arch of justice, by which he desired to knit together the whole human family." - "Mary Wollstonecraft",
Elizabeth Robins Pennell.
If this staging is removed before the keystone
is put in place, the entire structure falls in ruins. - "Studies in the Out-Lying Fields of Psychic Science",