A vertical component forming the side of a window frame, or a horizontal component forming the head or sill, provides structural support and a surface for attaching the window sash. It lines the opening in the wall, creating a stable and finished look. For instance, in a typical double-hung window, these components frame the opening in which the sashes slide up and down.
The accurate construction of these elements is crucial for proper window operation, weather-tightness, and overall energy efficiency. Historically, they were crafted from solid wood, often requiring meticulous joinery. Modern construction utilizes materials such as vinyl, aluminum, and composite, offering increased durability and resistance to the elements. Proper installation ensures the window is square and plumb, preventing drafts and reducing heating or cooling costs.