Which of the following is NOT considered a part of a window?

A threshold is typically associated with doors rather than windows. In the context of window components, the sill, header stop, and exterior sash are all integral parts of a window structure.

The sill is the horizontal bottom part of a window frame that supports the window and directs moisture away. The header stop is a molding that sits at the upper part of a window frame, helping to secure the window and prevent drafts. The exterior sash refers to the moveable part of a window that holds the glass pane, allowing it to open and close.

Since a threshold serves the purpose of bridging the gap at the bottom of a door, it does not play a role in the functionality or structure of a window. Therefore, it is accurately identified as not being a component of a window.