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Glossary of Terms
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A Astragal: The center portion of a double door, attached to the fixed or inactive door panel.

Awning window: A window unit whereby the bottom of the sash swings outward.
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Bay window: A composite of three 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.
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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.

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

Cladding: An aluminum material locked to the outside faces of many window and door 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.
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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. An example is the weatherstripping used between the frame and sash of a casement window.
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ENERGY STAR® : is a voluntary partnership between the U.S. Department of Energy, the U.S. Environmental Protection Agency and the fenestration industry. It is designed to assist consumers in the recognition of energy efficient products and promote the environmental and economic benefits of these products with the ENERGY STAR® label and other program activities. More information about ENERGY STAR® is available on our site by clicking HERE.

Extrusion: A form produced by forcing material through a die. Some window frames can be said to be clad with extruded aluminum.
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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.
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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.

Grids: A term referring to windowpane dividers or muntins, usually a type of assembly which may be detached for cleaning. Also called Grille's
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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.
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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.
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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.
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Knocked down: Unassembled window or door unit.
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Light: 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. Sometimes spelled 'lite.'

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

More about our LoE² Glass Window Products

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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, grille or grid.
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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.
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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 or R-Factor: Resistance to thermal transfer or heat flow. Higher R-value numbers indicate greater insulating value.
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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: 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.
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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.
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U-value or U-Factor: 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.

Unison lock: A casement locking system which secures the window at two locking points by operation of one handle. This is a feature that comes standard on all but the shortest Pella® brand casement windows.
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Vent Unit: A window or door unit that opens or operates.

Vinyl: A plastic material used by some window manufacturers for cladding or entire window units.
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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.

Windowpane divider: See muntin.
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Glossary of Terms

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