Pollock writes: "I find the location, orientation, and association of this building with nearby structures difficult to understand. As will be discovered below, the transverse vault that separates the north and south rooms was once a passage, but one that led nowhere on the west, where the building is squeezed against the substructural terrace of the Nunnery Quadrangle, and to no currently discernible point of interest to the east.
I believe the structure is best explained as having been erected as part of an earlier grouping of buildings, which may or may not have been completed, but in any case the design of which is now obscured by the great Nunnery Quadrangle. In its final stage, with the central passage closed at its west end, the structure clearly faced east and is more likely to be associated with the House of the Magician complex than with the Nunnery."
H.E.D. Pollock, "The Puuc: An Architectural Survey of the Hill Country of Yucatan and Northern Campeche, Mexico, p. 233
This arch, on the opposite side of the Northern Long Building facing the Nunnery Quadrangle, shows almost diagramatically how a Maya (false) arch was constructed.
The Maya arch lacked a keystone at the top and instead used L shaped "boot stones" to redistribute weight and keep the sides of the arch from collapsing inward. This type of arch was topped by a capstone, the flat stone joining the sides of the arch at the top.
At Uxmal and elsewhere, glyphic inscriptions and small paintings often decorate the capstones of interior vaulted rooms, and ancient graffiti is often found on the sides of vaults.