What does the word keystone mean?

Usage examples for keystone

  1. " 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 by Elizabeth Robins Pennell
  2. 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 by Hudson Tuttle