What is the masonry unit immediately under an opening such as a window or door?

The correct answer is a sill. In masonry construction, a sill is the horizontal member that is typically located at the bottom of an opening like a window or door. It serves as a support for the lower portion of the opening and helps direct water away from the structure to prevent moisture accumulation and damage.

The sill also plays a significant role in maintaining structural integrity by providing a surface for the wall mass above to rest upon. Properly constructed sills can enhance the durability and stability of the opening, ensuring that it withstands the weight and pressure of the materials above it.

Understanding the function of a sill is important in masonry work, as it influences both the effectiveness and longevity of the structure. Other components, such as a lintel which goes above the opening, serve different purposes in the overall construction.