The keystone's function is to hold the arch in compression.

• A. Provide aesthetic appeal
• B. Allow water passage
• C. Support the abutments
• D. Hold the arch in compression

Answer: (D)

Arches work because they transfer weight as compression along their curve. The keystone at the crown wedges into place and locks the voussoirs together, creating a continuous chain of compression that carries the load down to the supports. This locking is what keeps the arch stable under load, since masonry is strongest in compression and weakest in tension. The outward thrust at the supports is then resisted by the abutments, while the keystone’s main job is to hold the arch in that closed, compressive state. Aesthetic appeal or simply letting water pass are not the primary mechanical roles of the keystone, and while abutments matter for stability, they aren’t the keystone’s function.