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.

Air Latch
Latch mechanism on the interior face of the sash that retains the window in a partially open position for ventilation.

Apron
Inside flat trim member which is used under the sill at the bottom of the window.

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.

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 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.

Brick mould
A type of external casing around windows and doors used to cover jambs.

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 sash is hinged on the side and cranks outward, to the left or right.

Casing
Inside casing is a flat, decorative moulding that covers the inside edge of the jambs and the rough openings between the window unit and the wall. Outside casing (or brick mould, above) serves the same purpose, while it also can be an installation device through which nails are driven to install the window unit into the wall.

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.

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

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.

Conduction
Energy transfer from one material to another by direct contact.

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

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-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.

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

Extension Jamb
A board used to increase the depth of the jambs of a window frame to fit a wall of any given thickness.  Extension jambs butt up against the window and come out flush with the interior wall so the casing can be applied.

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

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

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

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 door
Hinged doors that swing inward upon opening.

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

Grille
A term referring to windowpane dividers or muntins, most commonly found between the glass today.
Also known as grids.

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.  Commonly found in basements.

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.

Knocked down (KD)
Unassembled window or door unit.

Lift
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 or lite
Glazing framed by muntins and/or sash in a window or door.

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.

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.

Muntin Bar
Any small bar that divides a windows glass. Also called a grille or windowpane divider.

Nail-fin
Windows with nail-fins are used on new construction homes for a fast, easy outside installation.

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.

Passive solar collector
Any glazed area in the walls or roof of a building pointed to the south to take maximum advantage of the sun's heat without a mechanical (or active) method of storage or distribution of the heat.

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

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.

Pane
A framed sheet of glass within a window.

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

Sash balance
A system of weights, cords and/or coiled springs which assist in raising double-hung sash and tend to keep the sash in any placed position by counterbalancing the weight of the sash.

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.

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.

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

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.

Sloped Sill Adapter
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.

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.

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. Standard on all doors and large fixed windows.

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.

Unison lock
A casement locking system which secures the window at two locking points by operation of one handle.

Vapor barrier
A watertight material used to prevent the passage of moisture into or through floors, walls and ceilings.

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

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

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.