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.

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.

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.

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.

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.

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.

Extrusion
A form produced by forcing material through a die.

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

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.

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.

Grille
A term referring to windowpane dividers or muntins, whether internal or external.

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.

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.

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.

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

Multi-Point Lock
A Casement locking system that uses one locking lever to engage and disengage up to three locking stations.

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.

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

Parting stop
In a double-hung window, a strip of wood applied to the jamb to separate the sash.

Power spacer
High density foam material that separates glass lites; used because of its high insulating properties.

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.

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.

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.

Super spacer
Foam material that separates glass lites; used because of its high insulating properties.

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.

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.

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.