The masonry that surrounds window apertures consist of 3 main components, the heads and cills, and the jambs. Jambs are the main vertical parts of a window surround, sited on the outside of the window structure. A head is a structural horizontal block that spans the gap between the two upright supports of the window aperture. Heads can be decorative and stuctural. The top part of the window. The head, which can also be called a lintel, is a single unit of masonry that supports the building above the window. The cill is the lowest part of a window, the bottom face of the cill rests on the wall below the window, and the window frame sits on top of it. Sloped to drain rainwater.
On larger windows, there are two more components that can be added to the structure, known as mullions and transoms. Transoms are horizontal parts of the window structure. They can separate glass from glass, or glass from another part of the aperture, like a door. A mullion is the vertical part of the inside of the window structure that makes a split between the sections of a window frame. Primarily for supporting the structures above, the mullion also provides structure for the windows themselves.