What architectural feature is primarily enabled by the keystone?

The keystone is a critical architectural element found at the apex of an arch, serving a pivotal role in the overall structure of the arch itself. Its primary function is to lock the other stones of the arch in place, providing stability and strength to the arch structure. The shape of the keystone is typically wedge-like, which allows it to distribute the weight of the upward forces equally to the adjoining stones, enabling the entire arch to bear loads effectively without collapsing.

In the context of the other options, while domes and vaults also employ similar principles, they are broader architectural forms that can consist of multiple arches combined together. A dome is essentially a series of arches rotated around a central point, and a vault refers to an extended arch, which may use keystones throughout, but the keystone itself is fundamentally a feature that specifically highlights the integrity of an individual arch.

Columns, on the other hand, do not rely on keystones. Columns function as vertical structural elements, supporting beams, arches, or ceilings, but they do not incorporate the same mechanism of support as arches do. Therefore, the specific architectural feature directly enabled by the keystone is the arch.