Apron: Inside horizontal trim located under the window stool at the bottom of a unit.

Argon: An odorless, colorless, tasteless, nontoxic gas that is six times denser than air. Replacing the air between two panes of glass with argon gas reduces temperature transfer, making the surface of the glass inside the house closer to the inside temperature.

Awning window: Hinged at the top, this window has a single sash that swings outward from the bottom.

Balance: Device for counterbalancing a sliding sash, usually associated with a double-hung window, so sash may be held open at any given position. Usually a system of cords, weights, springs, spiral devices or block and tackle hardware.

Bay window: A composite of 3 or more windows that project out from the wall. Usually consists of one large center window with two flanking fixed or operating windows at 30, 45, or 90 degree angles to the wall.

Bottom rail: Bottom horizontal member of a window sash.

Bow window: A composite of 3 or more windows in a radial or bow formation. Typically consists of casement windows both fixed and operating assembled at 10 degree angles from the wall.

Brickmold: Standard outside casing around the window to cover the gap between the window frame and the opening. Nails are driven through the molding to install the window to the framed opening.

Caming: Lead strips which bond small pieces of decorative glass in windows.

Casement: A window with side hinges that cranks outward from either the right or left.

Casement operator: A hardware device used to operate a casement window to any open position.

Casing: Molding or trim available in many widths, thicknesses and profiles applied to the frame around a window or door to cover the space between the window frame and wall.

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.

Circle top: A generic term referring to a variety of window units with one or more curved frame members, often used over another window or door.

Cladding: Usually an aluminum or vinyl material fixed to the outside faces of wood windows and doors to provide a durable, low-maintenance surface.

Condensation: The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a window glass or frame that is exposed to cold outdoor air. Is controlled by limiting the amount of humidity inside of a room relative to the outdoor temperature.

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

Crank handle: A handle that attaches to an awning or casement operator, used to open the venting window.

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

Double-hung window: A window with two vertically moving sashes, each closing a different part of the window.

Drip cap: Horizontal exterior molding to divert water from the top casing so water drips beyond the outside of the frame.

Extension jamb: A board used to increase the depth of the jambs of a window frame to fit a wall of any given thickness.

Extrusion: A form produced by forcing metal or vinyl through a die. Window and door frames are often clad with extrusions.

Fenestration: The arrangement, proportioning and design of windows and doors in a building.

Fixed light (also fixed sash): Window or sash which is non-operative or non-venting.

Frame: An enclosure or combination of parts which surround a window sash or door panel.

Geometric window: A fixed framed window made up of 2 or more angles (i.e., pentagon or trapezoid).

Glazing: The glass panes or lights in the sash of a window. Also the installation of glass in a window.

Glazing bead (also glass stop): Removable trim that holds glass in place.

Gothic-head window: Usually removable for easy cleaning, grilles give the appearance of a divided window pane.

Grille (or muntin bar): Usually removable for easy cleaning, grilles give the appearance of a divided window pane.

Head jamb (also head): All of the horizontal members that make up the top of the window or door frame.

Header: Supporting member or beam above window opening which transfers building weight above to the supporting wall structure on each side of the window.

Impact resistant glazing: Glass specifically manufactured to withstand impacts from airborne objects or forced entry. Usually a type of laminated glass often used in coastal areas impacted by hurricanes.

Insulating glass: A combination of 2 or more panes of glass with a hermetically sealed air space between the panes. The space may be filled with an inert glass such as argon.

Jamb: The vertical members at the side of the window or door frame.

Jamb depth: Width of the window frame from inside to outside.

Jamb liner: The plastic or metal track installed in the jambs of the window in which the window sash slide.

Keeper: The hook-shaped piece of hardware that it is mounted on the inside sash stile of a casement window in which the sash lock engages.

Laminated glass: Similar to the construction of car windshields, this technique sandwiches a piece of transparent film or plastic between two panes of glass. Typically used for safety reasons because of its resistance to shattering. Also reduce noise transmission to the interior.

Lift: A handle or grip installed on the bottom sash rail of a double-hung window to assist in the raising or lowering of the sash.

Light (also lite): A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in the upper and lower sash, as in 6-over-6.

Low-E Glass: A term used to refer to glass which has low-emissivity due to a film or metallic coating on the surface of the glass. Usually constructed of dual, sealed panes of coated glass filled with pure inert gas to block ultraviolet heat, for cooling purposes, while reflecting room heat back into the room for heating purposes.

Masonry Opening: The space in a masonry wall left open for the window or door.

Meeting rail (also lock rail): One of the two horizontal members of a double-hung sash which come together. A check rail.

Meeting stile: The vertical member in a pair of stiles, as in abutting casement windows.

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 vertical member (usually wood or metal) to structurally join two window or door units.

Muntin: Vertical or horizontal bars used to separate glass in a sash into multiple lights. Often called a grille.

Nailing Fin: A vinyl or aluminum extension attached to the frame of a window or door which creates a positive seal between the window and the framed wall. Acts as an additional barrier against air and water leakage. Screws or nails are fastened through the fin to hold the unit in the opening.

Operable window: Window which can be opened for ventilation.

Operator: A metal arm and gear attached to a window which allows for easy operation.

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

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

Parting stop: A vertical strip on each jamb that separates the sash of a double-hung window.

Picture window: Large fixed windows.

Rail: Horizontal member of a window sash or door panel.

Rough Opening: A framed opening in which the unit will be installed.

R-Value: The measurement of resistance to heat transfer in a material. The higher the R-Value, the greater the insulation value.

Sash: Framework of stiles and rails in which the glass of a window or door is set.

Sash lock: A lock applied to the window to pull the sash tightly against the frame (casement) or to pull the check rails together (double-hung) in order to seal the sash from weather and for security.

Seat board: A flat board cut to fit the contour of a bow or bay window and installed between the sill and the wall surface, providing a seat or a shelf space for plants, etc.

Shading coefficient: Decimal value which is the solar gain of a window, divided by the solar gain for a clear single-glass window of the same size. The shading coefficient of clear, double-glazing is about 0.85 to 0.9.

Side light: A fixed, often narrow glass window next to a door opening or window opening.

Sill: Horizontal member at the bottom of the window frame.

Single glazing: Use of single panes of glass in a window sash or door panel. Not as efficient as double glazing.

Single-hung window: Window similar to double-hung window, except the top sash is stationary.

Slide-by window: Windows which slide horizontally.

Stacked windows: Combined grouping of awning, casement, or non-operative windows to form a large glazed unit.

Stile: Vertical member of a window sash or door panel.

Stool: An interior trim on a window which extends the sill and acts as a narrow shelf. Often seen on double-hung window.

Stop: A wood trim member nailed to a window frame to hold, position or separate window parts.

Tandem lock: A locking system which secures the window at two locking points by the operation of one lever.

Tempered glass: Special heat-treated, high-strength safety glass which shatters into pebble-sized particles and not in slivers.

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

Thermal break: A thermal insulating barrier between two thermally conductive materials.

Transom: Small window located above a door or another window.

Transom joint: Horizontal member separating a door from a window panel above the door, or separating one window above another.

Triple glazing: Three panes of glass with two air spaces between, commonly consisting of an insulating glass with a separate storm sash. Also available in an insulating window in a single frame.

U-Value: Measurement of heat transfer through a given material. The lower the U-Value, the better the insulation value.

Venting unit: A window or door that operates or opens for ventilation.

Weather-stripping: A strip of resilient material applied to the perimeter of the sash and/or frame of a window or door to minimize the potential for water and air infiltration.

Windload: The force exerted on a surface by moving air.

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