Apron: A horizontal piece of trim underneath the sill.
Glass: Also called light or glazing. Four main kinds are available (see Window Glass Options).
Grille: Also called grid or muntin. Decorative cross pieces that divide the window into panes. Some grilles are suspended between the glass; others snap off for easy cleaning. Grilles for double-hung windows are named according to the number of squares they form (like six over six, as shown above). Not to be confused with mullions, which are vertical or horizontal pieces between adjoining window units.
Head: The horizontal piece that forms the top of the window frame.
Jamb: The pieces that form the top, sides, and bottom of the window frame (includes the head).
Lock: Most new windows come with locks, and there are upgrades, such as magnetic locks, that allow windows to lock automatically when closed.
Rail: The horizontal part of a sash.
Sash: The movable part of the window; the framework that holds the glass in place.
Sill: A piece that runs along the base of the window frame.
Stile: The vertical part of a sash.