What is the term for a sloping surface against which the ends of an arch rests?

The term that describes a sloping surface against which the ends of an arch rest is called skewback. The skewback is the angled surface that supports the arch and provides a stable base for its ends, allowing the weight of the arch to be appropriately transferred to the structure supporting it. This is crucial in masonry and arch construction, as proper alignment and support at the skewback help to maintain the integrity of the arch and prevent structural failure.

The other terms refer to different aspects of arch construction: a sill is typically a horizontal support at the bottom of a window or door; a keystone is the central stone at the apex of an arch that locks all the other stones into position; and an abutment refers to a structure built to support the lateral pressure of an arch or vault. Each plays a different role in masonry, highlighting the specialized functions of the various components involved in arch construction.