Astragal: The center member of a double door, which is attached to the fixed or inactive door panel.

Awning window: A window unit in which the bottom of the sash swings outward.

Bay window: A composite of three or more windows, usually made up of a large center unit and two flanking units at 30- or 45-degree angles to the wall.

Bow window: A composite of four or more window units in a radial or bow formation.

Brickmold: A type of external casing which frames windows and doors.

Casement window: A window unit in which the single sash cranks outward, to the right or left.

Casing: Molding of various widths, thickness and shapes applied to the framework of window and door units.

Check rail: On a double-hung window, the bottom rail of the upper sash and the upper rail of the lower sash, where the lock is mounted.

Cladding: An aluminum material locked to the outside facing of windows products to provide a durable, low-maintenance exterior surface.

Clerestory window: A venting or fixed window above other windows or doors on an upper outside wall of a room.

Cottage double-hung: A double-hung window in which the upper sash is shorter than the lower sash.

Dormer: A space which protrudes from the roof of a house, usually including one or more windows.

Double glazing: Use of two panes of glass in a window to increase energy efficiency and provide other performance benefits.

Double-hung window: A window unit that has two operable sashes which move vertically in the frame.

Drip cap: A molding placed on the top of the head brickmold or casing of a window frame.

Dual durometer: A material that has two or more levels of flexibility.

Extrusion: A form produced by forcing material through a die.

Fenestration: An architectural term referring to the arrangement of windows in a wall. From the Latin word, "fenestra," meaning window.

Finger-jointing: A means of joining individual pieces of wood together to form longer lengths. The ends of the pieces are machined to form a set of interlocking fingers, which are then coated with adhesive and meshed together under pressure.

Fixed: Non-venting or non-operable.

Flashing: A thin strip of metal or synthetic material that diverts water away from a window or skylight.

Frame: The enclosure in which window sash or door panels are mounted.

French hinged door: Hinged door(s) which have wider panel members around the glass.

French sliding door: A sliding door which has wider panel members around the glass, giving the appearance of a French hinged door.

Glazing: Glass in a window or door; the act or process of fitting with glass.

Glazing stop: The part of the sash or door panel which holds the glass in place.

Grille: A term referring to window dividers or muntins, usually a type of assembly which may be detached for cleaning.

Head: The main horizontal member forming the top of the window or door frame.

Header: A horizontal framing member placed over the rough opening of a window to prevent the weight of wall or roof from resting on the window frame.

Hopper: A window unit in which the top of the sash swings inward.

Insulating glass (IG): A combination of two or more panes of glass with a hermetically sealed air space between the panes of glass. This space may or may not be filled with an inert gas, such as argon.

Jamb: The main vertical members forming the sides of a window or door frame.

Jamb liner: In a modern double-hung window, the track installed inside the jambs on which the window sashes slide.

Knocked down (KD): Unassembled window or door unit.

Light (aka Lite): A separately framed piece of glass in a window or door. A traditional double-hung window, for instance, often has several lights divided by muntins in each sash. Such windows are described as six-over-six, eight-over-one, twelve-over-twelve, etc., to indicate the number of lights in each sash.

Low-E (Low emissivity) glass: A special type of glass having a transparent material fused into its surface which acts as a thermal mirror.

Masonry opening: The space in a masonry wall left open for windows or door.

Mortise: A slot or rectangular cavity cut into a piece of wood to receive another part.

Mortise-and-tenon: A strong wood joint made by fitting together a mortise in one board and a matching projecting member (tenon) in the other.

Mullion: A wood or metal part used to structurally join two window or door units.

Muntin: Applies to any short or light bar, either vertical or horizontal, used to separate glass in a sash into multiple lights. Also called a windowpane divider or a grille.

Palladian window: A large, arch-top window flanked by smaller windows on each side.

Panel: Usually refers to the separate panel or panels in a door frame.

Parting stop: In a double-hung window, a strip of wood applied to the jamb to separate the sash.

Rail: The top and bottom horizontal members of the framework of a window sash.

Rough opening: The framed opening in a wall into which a window or door unit is to be installed.

R-Value: Resistance to thermal transfer or heat flow. Higher R-value numbers indicate greater insulating value.

Sash: A single assembly of stiles and rails made into a frame for holding glass.

Sash cord: In double-hung windows, the rope or chain which attaches the sash to the counter balance.

Sash lift: A protruding handle screwed to the inside bottom rail of the lower sash on a double-hung window.

Sash weights: In older double-hung windows, the concealed cast-iron weights which are used to counterbalance the sash.

Sidelights (Sidelites): Narrow fixed units mulled or joined to door units to give a more open appearance.

Sill: The main horizontal member forming the bottom of the frame of a window or door.

Simulated divided light: A method of constructing windows in which muntins are affixed to the inside and outside of a panel of insulating glass to simulate the look of true divided light.

Single glazing: Use of single panes of glass in a window. Not as energy-efficient as double glazing.

Single-hung: A double-hung type of window in which the top sash is fixed or inoperable.

Stile: The main vertical members of the framework of a sash.

Stool: An interior trim piece on a window which extends the sill and acts as a narrow shelf.

Stop: A molding used to hold, position or separate window parts.

Tempered glass: Glass manufactured to withstand greater than normal forces on its surface. When it breaks, it shatters into small pieces to reduce hazard.

Tenon: A rectangular projection cut out of a piece of wood for insertion into a mortise.

Thermal break: The addition of a thermal insulating material between two thermally conductive materials.

Transom: A small window that fits over the top of a door or window, primarily for additional light and aesthetic value.

True divided light: A term which refers to windows in which multiple individual panes of glass or lights are assembled in the sash using muntins.

U-value: Rate of heat flow-value through the complete heat barrier, from room air to outside air. The lower the U-value, the better the insulating value.

Vent Unit: A window or door unit that opens or operates.

Weatherstripping: A material or device used to seal the openings, gaps or cracks of venting window and door units to prevent water and air infiltration.

Windload: Force exerted on a surface by moving air.