Home : Windows 101 : Glossary


Below are a few terms commonly used when discussing windows, their features and performance. These words and acronyms are standard in the industry and are used in most industry material about windows and patio doors.



American Architectural Manufacturers Association, a national trade association that establishes voluntary standards for the window, door and skylight industry.

Air Chambers

Small honeycomb spaces within the sash and frame which help to insulate and strengthen the window

Air Infiltration

The amount of air that passes between a window sash and frame. In windows it is measured in terms of cubic feet or air per minute, per square foot of area. The lower the number, the less air the window lets pass through.


American National Standards Institute, a clearinghouse organization for all types of standards and product specifications.

Argon gas

Argon is a safe, odorless, colorless, non-toxic, non-flammable inert gas that is commonly used in place of air between the glass panes of an insulated Low-E glass unit to reduce temperature transfer.

ASTM International

Formerly, the American Society for Testing and Materials, an organization that establishes material standards (including glass) and test methods. It has also produced a window installation standard.


A 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 for ventilation.

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Bay window

A composite of three windows, usually made up of a large center fixed unit and two flanking units at 30-,45- or 90- degree angles to the wall.


Mechanical device (normally spring loaded) used in single- and double-hung windows as a means of counterbalancing the weight of the sash during opening and closing.

Block frame window

A window manufactured with a frame that does not have any fins or flanges.

Bow window

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

Breather Tubes

Tube placed through air spacer and seal of insulating glass that allows unit to accommodate changes in pressure between time and location of manufacture and time and location of installation, where it is sealed. Usually used to accommodate changes in altitude between plant and job site. Also referred to as capillary tube.


A type of external casing which frames windows and doors.


A rubber material that seals the glass to the spacer, creating an airtight and watertight IG unit. Butyl has the lowest gas permeability of all rubbers.

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Cam Lock and Keeper

The mechanisms which pull the sash together when placed in the locked position.

Casement window

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

Center of Glass U- and R-values

The U- and R-values measured from the center of the glass to 2-1/2" from the frame.


Water vapor from the air deposited on any cold surface that has a temperature below the dew point. Sometimes a problem on cold window glass or framing that is exposed to humid indoor air.

Condensation Resistance Factor

A measure of the effectiveness of a window or glazing system to reduce the potential for condensation. The higher the condensation resistance factor, the more efficient the window and glazing system.


Energy transfer from one material to another by direct contact.


Heat transfer by currents that flow from a warm surface to a colder one.

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Design pressure (DP)

A measurement of the structural performance of a window or door. Usually specified as one-and-a-half times greater than necessary based on expected building, wind and weather conditions.

Double-hung window

A window unit that has two operable sashes which move vertically in the frame.

Double or dual glazing

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

Double strength glass

Glass between 0.115 and 0.133 inch thick. Also 1/8 inch.

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Egress window

Window designed to be large enough for a firefighter to climb in or a person to climb out of in an emergency. U.S. building codes require each bedroom of a home to have an emergency exit window, with minimum sizes specified.

Energy Star

A program sponsored by the U.S. Department of Energy that establishes minimum performance standards for windows to be recognized as energy efficient. Four different sets of standards for U-value and solar heat gain have been established for four different climate zones in the U.S. See the Association Directory for more information on the Energy Star Windows program. More stringent requirements are planned for 2009.


A form produced by forcing material through a die. Most window frames are clad with extruded vinyl or aluminum.

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Fixed window or picture window

Non-venting or non-operable window.

Flush or stucco fin

A replacement window with a fin flush with the exterior face of the frame and is used when the existing window frame id to be left in the opening. This is the most commonly used replacement of windows in a stucco home situation.


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.

Fusion weld

A term for a type of corner construction, used with vinyl and other types of windows and doors, in which a small amount of material on the ends of two pieces are melted or softened, then pushed together to form a single piece. This also is referred to simply as a welded corner

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

Glazing bead

A plastic or wood strip applied to the window sash around the perimeter of the glass.

Glazing stop or bead

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

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


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.


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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J-Channel frame

A window frame with a channel built-in to the side. This channel is designed to accommodate the ends of siding pieces to provide a finished appearance.


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

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Laminated glass

Two or more sheets of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for enhanced safety and security, as well as sound reduction.

Lift rail

A handle or grip installed on the bottom rail of the lower sash of a double-hung window to make it easier to raise or lower the sash.

Light (also lite):

A window; a pane of glass within a window. A number of sections within a glass unit that are divided by the muntin bars or grids.

Light shaft

An insulated shaft built to direct the light from a roof window or skylight through the attic to the room below.

Low-E glass

A common term used to refer to glass which has low emissivity due to a film or metallic coating on the glass or suspended between the two lights of glass to restrict the passage of radiant heat.

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Masonry opening

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

Meeting rail (also lock rail):

One of the two horizontal members of a double-hung sash which come together. A check rail.


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

Mortise lock

A lock fitting a mortise cavity in the edge of a door.

Mullion or mull

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

Multipoint lock

A locking system, operated with one handle, that secures a window or door at two or more locking points.

Muntin Bar

Any small bar that divides a windows glass. Also called grilles or grids

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Nail fin

An accessory component or integral extension of a window or patio door frame that generally overlaps the conventional stud construction and through which nails are driven to secure the frame in place.

Neat glass

A glass treated with a special coating. Currently, commercially available products feature a coating that uses the sun's UV rays to break down organic dirt through what is called a photocatalytic effect. The coating also provides a hydrophilic effect, which reduces the surface tension of water to cause it to sheet down the surface easily and wash away dirt.


National Fenestration Rating Council. A body that has established methods for rating and certifying the energy performance of windows, doors, skylights and other fenestration products.

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

Picture or fixed window

Non-venting or non-operable window. Also know as a fixed window.

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Rough opening

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


Resistance to thermal transfer or heat flow. Higher R-value numbers indicate greater insulating value. R-value is frequently used by the insulation industry and is the reciprocal of U-factor, a value more generally used in the window industry.

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

Sash rail

A protruding handle on the inside bottom rail of the lower sash on a double-hung window.

Seat board

A flat board cut to fit the contour of a bow or bay window and installed between the sills and the flat wall surface, providing a seat or shelf space.


Wood wedges (often wood shingles) used to level and plumb when securing the window or door unit in the rough or masonry opening in a square, level and plumb position during and after installation.


Narrow fixed units mulled or joined to door units to give a more open appearance.


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

Single glazing

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


A window resembling a double-hung, or vertically sliding window, with a fixed, non-operating top sash.

Slider window

A window in which the sashes move side to horizontally.

Sloped sill adapter, sill angle or sill extender

Used to cover the gap between the old sloped sill window and the new block frame window. It adapts a new window to the existing sloping sill.

SHGC (solar heat gain coefficient)

A rating, which is now generally replacing shading coefficient, measuring a window's ability to transmit solar heat. It measures both the solar radiation which is directly transmitted, as well as the solar radiation absorbed by the glass and subsequently transmitted. The lower a unit's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. It is approximately equal to the shading coefficient divided by 1.15. It is expressed as a number without units between 0 and 1.


The material used around the edges of a glass unit to separate the panes of glass.


a strip of material that fits into a groove on a screen frame to secure the screen fabric in place.


The main vertical members of the framework of a sash.

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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. Standard on all doors.


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

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Rate of heat flow-value through a building component, from room air to outside air. Also referred to as U-value. The lower the U-factor, the better the insulating value. U-factor, a rating more generally used in the window industry, is the reciprocal of R-value, a rating commonly used in the insulation industry.

Ultraviolet light (UV)

Invisible rays of solar radiation at the short-wavelength violet end of the spectrum. Ultraviolet rays can cause fading of paint finishes, carpets and fabrics, as well as deterioration of some materials.

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Vent or night lock

A small device located in the window frame or sash that restricts the opening of the sashes and allows for ventilation.


A generic term used for polyvinylchloride or PVC, an extruded material used for window and door manufacturing.

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Warm edge

A type of insulating glass construction using an airspacer offering lower thermal conductance than traditional aluminum spacer. Warm-edge IG units typically offer higher resistance to condensation and an incremental improvement in window energy performance.


Window and Door Manufacturers Association. Formerly the National Wood Window and Door Association, this trade organization has established many standards related to wood window and door products.


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


A drainage system built into the window frame to allow for the drainage of water to the outside.


Force exerted on a surface by moving air.

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