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.


Balancing: The raising and lowering of sash as well as holding its position at any desired level; sash activation.

Bar: A narrow rabbeted, horizontal or vertical sash or door member extending the total length or width of the glass opening; extends from rail to rail or stile to stile; often also called “TDL bar”, “SDL bar” or “Grid”; diagonal bars may extend from stile to rail or vice versa.

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.

Bevel: To cut to an angle, other than a right angle, such as the edge of a sash or door.

Beveled Raised Door Panel: A raised door panel with the edges of the raised face at an angle of approximately 30-45 degrees.

Bow Window: A composite of four or more window units projecting from the wall in a gently curved contour.

Box Bay Window: A composite of three windows, with flanking units projecting from the wall at a 90-degree angle.

: 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 Window: 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.

Casing (Moulding): Moulded or surfaced-four-sides pieces of various widths and thicknesses for trimming out door and window openings; casing may be classified as “exterior” or “interior” as far as window and exterior door frames are concerned.

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

Chamfer: A corner of a board beveled at a 45-degree angle; two boards butt-jointed and with chamfered edges form a “V” joint or right angle.

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.

Cope: To cut or shape the end of a moulded wood member so that it will cover and fit the contour of the sticking coping.

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.

Crossbuck: The panels of a door separated by intersecting diagonal rails arranged similar to the Roman numeral “X”; also sawbuck.

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.


D.L.O.: (Daylight Opening) The measurement across the face of any wood part that separates two sheets of glass.

Dado: A rectangular groove cut across the grain of a wood member; in contrast to plow; which is cut with or parallel to the grain.

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.

Door Bevel: The bevel on the stile edge (lock edge) of a door, usually 1/16″ for doors under 2″ in thickness, so that the door may swing free of the door frame; bevel is approximately 3 degrees towards door stop.

Door Panel: A sheet of thin lumber, plywood or composition material inserted into the frame formed by the stiles, rails and mullions of a door.

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

Double Hung Window: Consists of two sash, top and bottom, which (1) slide vertically past each other, (2) are joined by a meeting rail and (3) are held in any open position by means of weights or one of several types of balancing devices.

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)

Dutch Cut: The meeting point of the lower and upper section on a Dutch door.

Dutch Door: A door, usually exterior, with an upper and lower section that can be opened separately.


Entrance Door: A door on the front entrance of a structure; also “front” or “main” entrance door; may be single or in pairs.

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.

Extension Hinge (Casement Hinge): A hinge whereby an outswinging sash swings away from the frame to provide a space between the frame and the sash for washing or cleaning from the inside; space provided varies between 2 and 4 inches.

Extension Jambs: Flat parts which are nailed to the inside edges of the window or door frame, so that it will fit a wider wall.

Exterior Casing (Moulding): Casing to trim the exterior of window and exterior door frames, to serve as the boundary moulding for the siding material, and to form a rabbet with the blind stop or jamb for the screen, storm sash, blind, shutter or combination door; exterior casing is most commonly used on wood facing wood frame exterior walls; three pieces of exterior casing are used for each frame, namely two side and one head; also exterior or outside lining.

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


Fenestration: The placement or arrangement and sizes of the windows and exterior doors of a building.

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.

Fire Door: A solid-core flush door incorporating noncombustible material or fire-retardant chemicals to warrant specific fire ratings.

Fire-Rated Door: A door which has been constructed in such a manner that when installed in an assembly and tested it will pass ASTM E-152 “Fire Test of Door Assemblies” and can be rated as resisting fire for 20 minutes (1/3 hour), 30 minutes (1/2 hour), 45 minutes (3/4 hour) (C), 1 hour (B), or 1-1/2 hours (B). The door must be tested and carry an identifying label from a qualified testing and inspection agency.

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.

Flat Door Panel: A door panel consisting of a flat piece of plywood, solid wood or other material; contrasted to “raised door panel.”

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.

Flush Moulding: A moulding on the same level or plane as the wood member or assembly to which it is applied; a member is flush with another when they form an unbroken or even surface; in contrast to “raised” or “recessed” moulding.

Folding Door: One of two or more sliding doors hinged to move laterally in an opening; “accordion type” door; may be used for a folding partition; a complete unit for folding doors may be obtained consisting of doors with butts applied, track and guide hardware, door pulls and door frame (optional).

French Casement: Two casement sash each hinged on one stile and opening in the middle; the sash extend down to the floor and serve as a door to a porch or terrace; also Rabbeted Casement.

French (Hinged) Door: An interior or exterior door consisting of stiles, top and bottom rail and divided glass panels or lights; often used in pairs.

French Sliding Door: A sliding door consisting of stiles, top and bottom rail and divided glass panels or lights; giving the appearance of a French hinged door.

Flush Door: A door consisting of a core, crossbanding and flat face veneers or a core and flat face veneers only.

Full Bound Frame: (Usually a window unit) composed of frame on all four sides and no sill; “frame all around.”

Full Bound Sash: Sash having stiles and rails of same width; “same rail all around.”


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)

Grille: An ornamental item that visually divides a piece of glass into separate panes; snapped into the window or door sash from the inside, the grille replaces muntins and bars.


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)

Handing: A term describing the swinging direction of a window or door as one stands on the side of the door from which security is desired, namely the outside.

Headboard: 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.

Heat Loss: The Heat Transmission Coefficient (“U” Value) multiplied by the area of the door or window opening or other member of a structure.

Hip Raised Door Panel: A raised door panel with the edges of the raised face perpendicular.

Hollow-Core Flush Door: A flush door with a core assembly of strips or other units of wood, wood derivative or insulation board which supports the outer faces and with intervening hollow cells or spaces.

Hopper: An upside-down awning window, hinged at the bottom and opening inward from the top; many basement windows are this type. Also, frequently referred to as a ranch-type window.


Insulating Glass: Two or more (generally two) pieces, lights or panes of glass separated by a hermetically sealed air space 3/16 to 1/2 inch in width.

Interior Casing: Casing to trim the interior of window and door frames; three pieces of casing are required, namely two of side casing and one of head.


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.

Jamb: The top and two sides of a door or window frame which contact the door or sash; top jamb and side jambs.

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


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


Laminated: Layers of veneer or lumber bonded with an adhesive so that the grain of all layers is essentially parallel; contrasted to plywood, in which the adjacent layers are usually at right angles to one another.

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.

Leaded Glass: Small, usually irregular panes of glass, sometimes vari-colored, joined together by lead or zinc muntins and bars, and used primarily for decorative purposes.

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

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 (Lite): A framed opening in a sash or door containing a pane of glass (glass panel); light opening, lite; divided-light; cut-up.

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.

Lug: An extension of the stiles beyond the meeting rails of a sash, usually ogee-shaped; usually sawn ornamentally on the inside of the stile; “ogee lug”; often found on single or double hung windows; also “sash lugs.”


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.

Millwork: A term to describe those products which are primarily manufactured from lumber in a planing mill or woodworking plant; includes mouldings, door frames and entrances, blinds and shutters, sash and window units, doors, stairwork, kitchen cabinets, mantels, china or corner cabinets and porch work; woodwork.

Mortise: A slot or rectangular cavity cut into a piece of wood to receive another projecting (tenon) 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.

Moulding: A relatively narrow strip of wood, usually shaped to a curved profile throughout its length; used to accent and emphasize the ornamentation of a structure and to conceal surface or angle joints.

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 TDL bar.


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

Non-Prefit Door: A door requiring further fitting prior to being hung; as opposed to “prefit door.”


Open Sash: A sash in which the glass has not been installed.

Operator: A crank-type metal arm and gear which allows for opening a window which swings outward; a jointed, metal arm keeps the window open at any position.

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

Ovolo: A convex profile; usually a quarter-section of a circle and similar to the profile of “quarter-round.”


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

Panel: A wood surface within a surrounding frame; all panels have structural frames, the interstices of which are filled with sheets or fields called “panels”; the frame is necessary for adequate strength only with the panels occupying considerable more area than the frame; the panel may be raised above or recessed below the surrounding frame and set off from it by moulding or other decorative treatment;.

Panel Door: Consists of stiles, rails and one or more panels, the stiles and rails forming the frame around the panel; “stile and rail” door.

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.

Plow: A rectangular groove or slot of three surfaces cut parallel or with the grain of a wood member; in contrast to “dado”, which is cut across the grain.

Prefit Door: A door not requiring further fitting or sizing upon installation; “prefit” doors have eased edges and “skid blocks.”

Prehung Door Unit: A precut and assembled unit consisting of a wood door with preparation for lock hardware that is hung on hinges in a wood frame; the wood frame includes the one-or two-piece jamb adjustable or as-ordered width as well as the door stop moulding and casings (trim); also hinged interior wood door unit; door units other than conventionally hinged are also available.

Prime Coat: The first coat of paint in an application that consists of two or more coats; also refers to the paint used for such an initial coat.

Putty Glazing: Glazing by use of putty; the glass is inserted into the putty rabbet, glazier’s points driven and the putty or glazing compound applied to the putty rabbet.


Quarter Round: Shaped as a quarter-circle.


Rabbet: A rectangular cut consisting of two surfaces cut on edge of a member parallel with the grain; a rabbet has two surfaces and a “plow”.

Rabbeted Jamb: A jamb with a rabbet on one or both edges; if both edges, “double-rabbeted jamb”; if one edge, “single-rabbeted jamb.”

Rabbeted (Pair): A pair of sash or doors with rabbeted meeting stiles to prevent swinging through; each sash or door has its meeting stile rabbeted on an opposite edge or corner.

Rabbeted Casement: Two casement sash each hinged on one stile and opening in the middle; the sash extend down to the floor and serve as a door to a porch or terrace; also French Casement.

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.

Raised Door Panel: A door panel whose face is raised above the panel edges which are moulded and shaped to fit in the grooves of the door stiles and rails; contrasted to “flat 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.

Raised Moulding: An applied moulding which partly covers or extends above the face or surface of an assembly.

Rough opening: The unfinished opening formed by the framing members (the top one is the “header”, the side ones are the “jack studs”, and the bottom one is the “rough sill”; also rough openings in brick walls are know as “masonry openings”) 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.

SDL (Simulated Divided Light): A method of constructing windows in which muntins are affixed to the inside and outside of a panel of insulating glass with an element placed in between to create the appearance of glass that is separated into small panes;  to simulate the look of true divided light (TDL).

Seatboard: 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.

Segment Head: A door or window head in the form of the arc of a circle.

 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.

Sill Horn: The extension of the “lip” of a window sill to the outside edge of the casing, brick moulding or sometimes wide blind stop or blind stop extension; the extension of a door sill beyond the frame.

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.

Sliding Door: A door which slides in a horizontal direction parallel to wall of the structure

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.

Solid-Core Flush Door: A flush door consisting of a core of solid wood blocks or strips with crossbanding and face veneers or face veneers only.

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.

Store Door: Similar to a French door with a wide bottom rail and usually glazed with one glass panel or light.

Strike Plate: A metal piece mortised into or fastened to the face of a door frame side jamb to receive the latch or dead bolt when the door is closed.

Stucco Moulding: Moulding similar to “brick moulding” except for the inclusion of a groove to receive and hold the “stucco”; the groove prevents plaster cracks where the moulding and stucco meet.

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

Sweep: A rubber or vinyl strip applied to the bottom of a door to create an effective seal against the sill (threshold).


TDL (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.

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.

Threshold: A wood or aluminum member, bevelled or tapered on each side, and used with exterior or interior door frames; classified as “interior” or “exterior”.

Interior Threshold: A threshold, symmetrically bevelled, which when installed provides door clearance for carpeting, ceramic tile and other floor coverings.

Exterior Threshold: A threshold non-symmetrically bevelled (the more gradual and longer bevel facing the exterior) which, when secured to the exterior door frame sill and/or finished floor, prevents water from driving under the door.

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 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.

Veneer: A thin sheet or layer of wood, usually rotary cut, sliced or sawn from a log, bolt or flitch; thickness may vary from 1/100 to 1/4 of an inch; also skin, ply, veneer ply.

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

V-Groove: A V-shaped groove cut into the surface of a wood member for decorative purposes; V-grooving is often done on the exterior surface or face of door panels or solid flush doors.


Weatherstrip: Variously shaped metal, vinyl plastic or moulded-fibre strips which fit tightly against sash or door frame parts to prevent air infiltration through cracks.

Windload: Force exerted on a surface by moving air.

Windowpane divider: See Muntin.

Wire Glass: Flat, rolled glass having a layer of meshed wire completely imbedded in the sheet; used in commercial applications.

Wood Stop Glazing: Glazing whereby a thin layer of putty or glazing compound is placed in the putty or glass rabbet, the glass pressed into the “bed” and secured by wood stops.