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.

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.

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

Balance System
Device for holding vertically sliding sash in any desired position through the use of a spring or weight to counterbalance the weight of the sash.

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.

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

Breather Tubes
A small metal tube that is placed into an insulated unit's spacer to equalize pressure differences. Breather tubes can allow moisture to enter into the insulating unit. Mostly used in high altitude locations.

Butyl
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.

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.

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

Cladding
Any material locked to the outside faces of doors and windows to provide a durable, low-maintenance exterior surface.

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

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

Fixed window
Non-venting or non-operable window. Also known as picture window.

Foam Spacer
Foam material placed in the airspace of the insulating glass in a window to enhance the appearance and improve the performance of the window.

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

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

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.

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.

Meeting 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.

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 grids. 

Pane
A framed sheet of glass within a window.

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

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.

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

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

Solar gain
The process of providing a net heat gain within a structure, over and above the normal heat loss, by passive collection of the sun's heat through windows and other glazed areas.

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 and large fixed windows.

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

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.

Vinyl
A plastic material used for cladding or entire window units.

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