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Definitionsomething of importance upon which other things depend
architecture or masonry: the central building block at the top of an arch or vault (providing pressure to lock the other blocks into place)
- The keystone of the arch crashed almost on their heels, and the wall above crumbled, and fell in ruin.6.1 -- Book 6 Chapter 1 -- The Tower of Cirith Ungol (99% in)
- At last they came out of shadow to the seventh gate, and the warm sun that shone down beyond the river, as Frodo walked in the glades of Ithilien, glowed here on the smooth walls and rooted pillars, and the great arch with keystone carven in the likeness of a crowned and kingly head.5.1 -- Book 5 Chapter 1 -- Minas Tirith (23% in)
There are no more uses of "keystone" in The Return of the King.
Typical Usage (best examples)