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In unit-masonry (e.g., stone block or brick) construction, the keystone is the central stone which completes the construction of an arch and permits it to carry vertical loads. The keystone is wider at the top than at the bottom, so that it acts like a wedge which uses the weight placed on it to force the other blocks forming the arch together, thus preventing them from falling.
This is an authentic handmade miniature reproduction in metal and is dated from around 1820.
(photo courtesy of rosedalefigures.com)