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

Argon gas: A colourless, odourless gas used to fill the airspace between insulating glass. The addition of Argon greatly increases the insulating performance of the Low E.

Astragal: A moulding attached to the non-primary (fixed or inactive) side of double doors. Locks into place at the top and bottom using flush bolts to prevent the door from opening when you open the primary door.


Bay window: A composite of three or more 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.

: Outside casing around window to cover jambs and through which nails are driven to install the window.


Casing:  Inside casing is a flat, decorative molding which covers the inside edge of the jambs and the rough openings between the window unit and the wall.  Outside casing (or Brick Mould) serves the same purpose, while it also is an installation device through which nails are driven to install the window unit into the wall.

Casement: A window that is attached to its frame by one or more hinges. They are hinged at the side, and are opened with a crank or cam handle.

Center line: the measurement from the bottom of the unit to the top of the radius.  (See Radius Terminology Diagram)

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.  Or, on a sliding window, the vertical sash rails that overlap when the window is closed.

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: A window near the top of an outside wall.

Cripples: The short 2 x 4 members used to frame under the sill or above the header in a rough opening for a window in a frame wall.

Curb: A watertight wall or frame used to raise slope glazing above the surface of the roof as a preventive measure against water leakage from melting snow or rain run-off.


Dormer: A space which protrudes from the roof, usually including one or more windows.

Daylight: The amount of visible glass (no obstructions) on a window.

Direct glaze: Glass which is directly put into the frame of a door. Typically used for a door sidelite, which will provide a wider viewing area.

Double rafter: The doubling (side by side) of the roof members to reinforce an opening in the roof for a slope glazing installation.

Double (Dual) glazing: Use of two panes of glass in a window to increase energy efficiency and provide other performance benefits.

Drip cap: A molding placed on the top of the head brickmould or casing of a window frame.

Drop: The measurement from the top of the unit to the spring line.  (See Radius Terminology Diagram)


Ellipse: Made using two or more radii.  (See Radius Terminology Diagram)

Envelope: The building envelope is all of the elements of the outer shell that maintain a dry, heated or cooled indoor environment and facilitate its climate control. Building envelope design is a specialized area of architectural and engineering practice that draws from all areas of building science and indoor climate control.

Eyebrow: A true radius but not a true half circle.  (See Radius Terminology Diagram)


Fenestration: An architectural term referring to the arrangement of windows in a wall.

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 metal or plastic strip attached to the outside of the head or side jambs to provide a weather barrier, preventing leakage between the frame and the wall.

Flush bolts: Found on the astragal of a door. They lock the non-primary door in place, or will unlock the door so that it can be opened.

French hinged door: Hinged doors 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.


Gable: The part of a wall that encloses the end of a pitched roof.

Glazing: The glass panes or lights in the sash of a window.  Also the act of installing lights of glass in a window sash.

Glazing bead: A vinyl or wood strip applied to the window sash around the perimeter of the glass on the outside to hold the glass in place.

Glazing compound: A pliable substance applied between the window sash and the lights of glass to seal against the elements and sometimes to adhere the glass to the sash.

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

Gothic: A pointed arch, especially one having only two centers and equal radii.  (See Radius Terminology Diagram)


Half Circle: A term that means the height is exactly half the width. A true half circle is a true radius, but a true radius does not have to be a true half circle, ex. Eyebrow. (See Radius Terminology Diagram)

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

Head board: A flat board cut to fit the contour of a bow or bay window and installed between the head jambs and the flat wall surface to finish off that area which would normally be ceiling.

Header: A heavy beam extended across the top of the rough opening to prevent the weight of wall or roof from resting on the window frame.

Hopper: A window with a top sash that swings inward.


Jack stud: Framing members, generally 2 x 4’s, which form the inside of the window or door rough opening.  They run from the sole plate to the header, which is supported by them.

Jambliner: Metal or plastic covering the inside surface and head jambs of sliding windows.


Keeper: The protruding, hook-shaped part of a casement window lock, which is mounted on the inside surface of the sash stile.


Laminated veneer lumber (LVL): A composite product manufactured from multiple thin layers of veneer that are aligned with the length of the finished lumber.

Laminated glass: A type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer of plastic between its two or more layers of glass.

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 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. The primary function is to reduce the U-value by suppressing radiative heat flow. Secondary feature is to block radiation to impede heat gain. There are two types of low-E; a hard coat and a soft coat.


Masonry openings: The opening in a masonry wall to accept a window or door unit, the same as a rough opening in a frame wall.

 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 on board and a matching projecting member (tenon) in the other.

Mullion: The vertical or horizontal divisions or joints between single windows in a multiple window unit.

Mullion casing: An interior or exterior casing member to cover the mullion joint between single windows.

Multipoint lock: Locks in multiple spots to enhance security and durability.

Muntin: A short bar used to separate glass in a sash into multiple lights.  Also called a windowpane divider or a grille.


Nail fin: The part of a window that gets nailed to the studs of a home.


Operator: A metal arm and gear which allows for easy operation or closing of projecting windows.

Outer frame member: The exterior protruding portion of a window frame which has no exterior casing.


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.

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 or storage or distribution of the heat.

Picture frame casing: The use of casing on all four sides of the interior of a window, replacing the stool and apron at the sill.  Also known as full-bound casing.

Pitch: The pitch of a roof is the degree of the inclination upward from horizontal or flat.  It may be expressed in degrees or as the ratio of the number of inches it rises in each 12 inches of horizontal span: 4/12 means the roof rises 4″ in every foot of horizontal span.

Pivot: A mode of operation for ventilating windows which generally means the sash pivots on a central axis and turns 90 or more degrees.


Rafter: Structural members of a roof which support the roof load and run from the ridge to the eaves (overhang).

Rails: The horizontal members of a window sash or door panel.

Rafter: Structural members of a roof which support the roof load and run from the ridge to the eaves (overhang).

Rake: A window with uneven sides, often used to mimic roof lines

Rough opening: The framed opening without any casing or finishing. Where the window or door is to be installed. Openings are larger than the size of the unit to allow shimming and insulating.

Rough sill: The horizontal framing member, usually a 2 x 4, which forms the bottom of the rough opening.  It is toe-nailed into the jack studs and is supported by cripples.



Sash: The part of the window that surrounds the glass and holds the glass in place in the window frame.

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 lock: Generally, a cam-action type lock applied to the check rails of a sliding window or at the open edges of a projecting window to pull the check rails tightly together or to seal the sash tightly to the frame, both for security and weather-tightness.

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.

 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.

Tall, narrow, fixed or operating sash on either or both sides of a door, built in the same frame, to light an entryway or vestibule.

Sill: Horizontal member that forms the bottom of a window frame.

Sill course (soldier course)The row of brick, cement blocks or stones laid across the bottom of a masonry opening which lie under the outside edge of the window sill.

Simulated divided light (SDL): 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.

Slope glazing: Any glazed opening in a sloped roof or wall, such as a stationary skylight or fully operable roof window.

Solar heat 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.

Sole plate: The bottom horizontal member in a frame wall.  Usually either single or double 2 x 4’s.  It is nailed to the deck or rough floor and the studs are nailed into it.

Spacer: The spacer bar is bonded to the glass panes via a primary and secondary seal, creating an airtight cavity which is filled with air or gas.

Spring Line: The point where a radius or curved portion meets straight vertical legs. (See Radius Terminology Diagram)

Stile: The vertical side member of a window sash or door panel.

Stool:  Inside horizontal trim member of a window sash or door panel.

Stop: A wood trim member nailed to the window frame to hold, position or separate window parts.  The stop is often molded into the jamb liners on sliding windows.

Stud: Vertical wood framing members which form a frame wall.  In normal construction these are 2 x 4’s about 8′ long.


Tempered glass: Float glass heated and cooled rapidly in a controlled environment, making the glass several times stronger. When it is broken it yields small pebble-like fragments.

Tenon: A rectangular projection cut out of a piece of wood for insertion into a mortise.

Transom:  A smaller window above a door or another window.  A transom joint is also the horizontal joining area between two window units which are stacked one on top of the other.

Transom double-hung: A double-hung window in which the upper sash is shorter that the lower sash.

Triple glazing: A sash glazed with 3 lights of glass, enclosing 2 separate air spaces.

True divided light (TDL): A term which refers to windows in which multiple individual panes of glass or lights are assembled in the sash using muntins.

True radius: A term which means that the arc is part of a circle.  The height must be less than or equal to half the width of the unit.


U-value: A measure of heat transmission through a wall or window.  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.

Venting unit: A window or door unit that opens or operates.


Windload: Force exerted on a surface by moving air.

Windowpane divider: See Muntin.