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A vertical member on the inactive panel of a pair of doors ("French doors") or a pair of windows ("French windows) to weatherproof the joint between the two panels.



The structural attachment of a window or door frame to the building. Windows and doors may be anchored directly with fasteners such as nails or screws, or with installation clips such as strap anchors or angles.


Capillary Tube

A very thin tube installed in the spacer(s) of an insulating glass unit to prevent excessive glass bowing due to pressure changes, such as installation of glass at an altitude significantly higher than the altitude at which the unit was made.



The interior trim between the window and the wall. Casing can refer to moldings or to the combination of materials, including paneling and moldings, that are used to finish the wall to window or wall to door joint.


Combination Window

A window that has both fixed and operable elements, or that is divided with mullions.



A product that allows people to enter and leave a room or a building. A door can be as simple as a single hinged panel, or an assembly of hinged or sliding panels with fixed sidelites and/or transoms.



Exit, usually referring to an emergency fire exit. Building codes may require certain windows to provide openings large enough to allow persons to escape from a room in case of fire. Egress requirements differ from one code jurisdiction to another.


Fin or Flange

Most windows and many doors are available with mounting flanges to make it easier to install the products into walls. A flange may be an integral part of the window frame or a component added to it. Depending on the installation method, flanges may be used to seal the window to wall joint or to attach the window to the wall.


Fixed Window

A window that cannot be opened for ventilation.



Rigid or flexible sheet material used to divert water away from critical building joints. Flexible flashings are sometimes used under the exterior finishes. Rigid flashings of painted steel or aluminum are used at the building exterior.



The assembly of lineal members that holds the sashes and glass lites together in a single unit that can be handled. Large windows or doors may be assembled from several frames.



The transparent material, typically an insulated glass panel in a window frame (noun) or the act of fitting glass panels into a window frame (verb).



The top of a window or door. The head can refer to either the top member of the window or to the top surface of the rough opening.


Insulating Glass

Glass panels made from two or more panes of glass separated by a spacer and sealed at the edges. The spacer contains dessicant, a material to absorb small amounts of moisture that may leak past the seals. Energy efficient insulating glass units (IGUs) typically have one or more Low-E (low emissivity) coatings to reflect heat, "warm edge" spacers that resist heat transfer, and an inert gas such as argon in the gap. Together these features reduce the heat loss through the unit.


Heel Size (H.S.)

The heel dimension is the edge-to-edge overall measurement of the part of a window or a door frame that fits into the rough opening. It is the frame size as measured from the room side of the window. (The "heel" of a window is the side of the window that faces the interior, and the "toe" is the side that faces the exterior.) Some window or door products have flanges or protrusions that are meant to lie against the outer face of the wall, but the window is not measured at the flanges or protrusions. Heel dimensions are always expressed as width x height.



The sides of a window or door. Jambs can refer to the vertical edge framing members of a window or to the vertical surfaces of the rough opening.



The glass panel or insulating glass unit in a window or door. See also Sidelite and Transom.



A vertical or horizontal framing member that separates the lites of a window.


Operable Window

A window that can be opened by sliding, tilting, or swinging on hinges.


Picture Window

A fixed window, usually large, for enjoying a view of the outdoors.


Rough Opening (R.O.)

The opening into which a window or door is installed. The rough opening is always larger than the window's heel size.


Rough Opening Size (R.O.S.)

The R.O. size refers to the rectangular dimensions of the opening into which a window or door will be placed. Most window and door products are rectangular and most windows are built to rectangular measurements even if the hole is imperfectly framed. To correctly measure the R.O. you first need to determine whether the sill is level and the jambs are plumb (perfectly vertical). If they are not, the measured width and height need to be reduced to account for the deviations from level and plumb. R.O. dimensions are always expressed as width x height.



The part of the window or door that opens.



On a stud framed wall, the structural paneling nailed or screwed to the exterior surface of the studs.



The glass lite beside a door.



The bottom of a window or door. In the case of windows (fixed or operable) the sill can refer to the bottom framing member of the window or to the bottom surface of the rough opening on which the window sits. In the case of doors the sill can refer to the bottom surface of the rough opening on which the door sits, to the bottom framing member of the door, or to a piece of trim fixed to the bottom framing member or to the floor.



A logical term for the line that divides the interior from the exterior at a door. A threshold can also be a trim molding installed on the floor under a door.



The area above a door or window sash. The transom can refer to the glass lite above the sash or to the horizontal framing member between the sash and the lite above.

Triple Glazed Window or Door

A triple glazed window or door, also known as triple pane window or door, is a window or door with three lights (or panes) of glass. The ideal width of the two airspaces should be between 3/8" - 5/8" thick for optimal thermal performance.



A glazed product installed in a wall to let in light, fresh air, or grant a view. Windows may be fixed or operable.