Part of speech: noun

The uppermost and completing stone of an arch.

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Usage examples "keystone":

  1. 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.
  2. " 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.
  3. 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", Hudson Tuttle.