What is the last stone set in place in an arch called?

The last stone set in place in an arch is called the keystone. This stone is crucial because it is the final piece that locks all the other stones of the arch into place. The keystone is often larger and more prominently shaped than the other stones, and it is designed to distribute weight evenly from the top of the arch down through the arch's structure. This distribution helps to maintain the structural integrity of the arch, allowing it to support loads above it while remaining stable and effective in its architectural function.

The other terms refer to different components of an arch or related structures: the base stone serves as the foundation from which the arch rises, the pinnacle stone typically refers to a decorative element at the top of a structure, and the spandrel stones are the wedge-shaped stones located at the curves of the arch that help to fill the area between the arch and a rectangular frame. These components play their own important roles, but they do not hold the same significance as the keystone in the functioning of the arch itself.