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Air Infiltration- The amount of air that passes between the window frame and the sashes or glass.  Measured in cubic feet per minute per square foot of frame opening.

Airspace Grilles- Decorative aluminum bars sealed between two panes of glass.

Anodized Finish- A very durable finish that is permanently etched into extruded aluminum.

Argon Gas- A colorless, odorless, inert gas that is inserted between panes of glass to improve thermal properties.

Astragal- A vertical component of a two panel opening, attached to one panel that allows the two panels to lock together.

Awning Window- A style of window that swings out at the bottom


Balance- A mechanism that holds an operating sash open in any position and allows the sash to be operated with minimal effort.

Bay Window- A three sided window that projects out from the home usually with a picture window and two operating flankers.

Bow Window- A series of four or more windows gently angled to create a curved shape that projects out from the home.

Brickmould- A common exterior decorative trim


Caming- The ornamental material that divides and holds together decorative glass in place.  Typically made from lead, brass or zinc.

Casement Window- A side hinged window that usually cranks open to the exterior.

Casing- Usually refers to interior trim around a window or door.

Caulking- A mastic material for sealing joints or cracks to prevent air or water leaks.

Check Rail (Meeting Rail)- The horizontal sash members on a double- or single-hung tilt window or the vertical sash members on a slide/by window that come together when the windows are closed.

Clad (or Cladding)- Refers to the portion of a wood window or door that’s covered with aluminum or vinyl on the exterior side of the frame or sash.

Condensation- The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a window glass or frame that is exposed to cold outdoor air.

Cottage-Style Single Hung or Double Hung- A single hung or double hung window with a bottom sash that is taller than the top sash.  Sometimes called Oriel.


Double Strength Glass- Sheet glass with a thickness of 1/8" or 3mm

Drip Cap- A horizontal molding to divert water from the top casing so that the water drips beyond the outside of the frame.

Double-Hung- A window where both the upper and lower sash slide up and down to allow air into the room.


Egress Window- See Emergency Escape and Rescue Openings

Emergency Escape and Rescue Openings (Formerly known as Egress Openings)
A window or door large enough for a person to climb out and/or for rescue in case of  emergency per local building code requirements.  Please consult Knebel Windows regarding windows that meet these requirements.

Extrusion- Vinyl or aluminum material  (aluminum is stronger) that has been moulded into a specific shape during a heating process.


Frame- The surrounding outside portion of a window or door composed of the head at the top, the side jambs and the sill at the bottom

Flanker- A window or door joined beside another window.  Flankers are the windows on either side of a bay window

Frame Size (also known as Box Size) Measurement of a window or door unit from the outside-of-frame to outside-of-frame, excluding the exterior trim.

French Casement- Two casement windows that have a double-wide opening because there is no center mullion between them but, instead, an astragal  is attached to one of the sash.

French Door- Two door panels that have a double-wide opening because the center mullion is replaced with an astragal attached to one of the door panels.

Fusion Welding- The process of mitering and heating the frame and sash corners of vinyl windows.  This fuses the corners together for exceptionally strong joints and permanently  sealed corners.


Garden Window- A traditional bay window combination with a direct set of window that’s flanked by operating or stationary casement windows placed at a 90-degree angle to the interior wall.  This particular bay window combination has an overhead glass pane that allows sunlight to flow directly into the window area.

Glazing- The installation of glass in a window opening; also refers to the fenestration or windows and putty on older wood windows.

Glazing Bead- (Glass Stop)-(Wood Stop)-(Sill Beard)- A removable trim that holds glass in place.

Green Building
- The ‘Green Movement’ brings up many concerns about energy efficiency, as well as company philosophy and practices.

Grilles- A decorative pattern on a window or door consisting of horizontal and/or vertical  bars that divide the larger sheet of glass into smaller panes.  Grille types include Simulated Divided Lite (SDL), True Divided Lite (TDL), Grilles In Airspace (GIA) or perimeter grilles and stick grilles.

Grilles in Airspace- Also referred to as airspace grilles or grilles between glass, these are aluminum muntin bars sealed in the insulating airspace between the two panes of glass, making cleaning easier.

Grid- See Grille

Grille- A grid that visually divides a window into panes.


Handleset- The hardware portion of a door consisting of a core, turning mechanisms,  a handle or knob, a strike plate and a lock plate.

Hardware- The locks, crank handles and hinges on windows and doors used to operate and secure them.

Hinged Patio Doors- a patio style door featuring two panels with hinges on the outer sides that allow the door panels to swing open either inward or outward from the center.  Center hinged, or French patio doors feature one fixed and one operating door panel with hinges between the panels to allow one panel to swing open over the other.

Hopper Window- A window that is hinged at the bottom and tilts in from the top for ventilation.  Hopper Windows are often used in basement or garage applications.


Infiltration- Leakage of outdoor air into a house, such as through cracks around sash or window frame.

Insulating Glass (also known as IG)- Glass units constructed of two or more glass panes separated by a hermetically sealed space.


Jamb Extension- An extra window frame piece added to the jamb that extends the frame to match the precise thickness of a wall.

Jamb Liner-  The tracks located inside the jambs of a double and single hung window on which the sash slide up and down.

J Channel- An accessory for vinyl windows and doors that snaps into the exterior accessory groove.  It is designed to provide a matching channel for the installation of siding.  It is sometimes integrated into the frame as well.


Laminated Glass- Two or more layers of glass bonded together using heat and pressure, with inner layers of transparent plastic.  Used as a safety and security device, much like the windshield of a car.

Lite- Refers to a single pane of glass or, in some cases, an entire glass unit.

 A door’s complete lock system consisting of the lock mechanism, knobs, keys, plates, strikes and other accessories.

Low E Glass- A term meaning low ‘emissivity.’ Glass with a microscopic coating applied to one or more surfaces to provide greater energy efficiency and UV protection that helps reduce the fading of artwork, curtains, furniture, rugs or floors in a home.


Meeting Rail- One of the two horizontal members of a double-or-single hung sash which come together.

Millwork- Wood products and mouldings made in a wood working plant.

Mullion- The stationary frame piece that separates mulled window or door units.  Being joined by mullions makes multiple window or door units appear to be one.

Mull (Mull or Mulling)- The actual piece used to attach two or more window and/or door units together to form a combination.  The process of joining the units to each other.

Muntin Bars- The narrow, horizontal and/or vertical bars that form the decorative grille pattern on a window or door.


Nailing Fin- A leg of the extended aluminum or vinyl exterior of clad units that forms a 90 degree angle to the window and is positioned flush against the sheeting of the building.  Clad windows  are easily secured in the rough opening by nailing through the fin into the sheeting and jack studs of the rough opening.


Obscure Glass (also known as pattern glass) Glass with a rough surface so that while it remains translucent,  it offers the privacy of an obscure view.

One-Lite- Usually refers to a single glass unit or picture window.

Operable/Operating Panel- A window and/or door panel that can be opened and closed. (Also known as ‘venting.’)

Operator Handle- The piece of window hardware  used to crank the sash open on a casement or awning window.


Pane- A single piece of glass within a window or door

Parting Stop- Trim piece that separates two  sash or panels.

Picture Window- A large, stationary window used to capture wide views of exterior landscape.  Can be combined with other functional windows  such as double hung or casements, and/or topped with  awning windows for enhanced views.

Plexiglas- Brand name of sheet acrylic plastic.  It can be used in windows  or doors and qualifies as a safety glazing.  It scratches easier than glass.

PVC- (Polyvinylchloride) See UPVC


Replacement Sash Kit or Sash Kits- A kit that allows existing double hung window sash to be replaced with modern window sash, complete with spring loaded counterbalances and easy sliding jambliners.  The replacement sash offer energy efficiency along with a convenient tilt-in feature that makes cleaning easier.

Retractable Screen- A mesh fiberglass exterior piece on a window or door that rolls down from a concealed aluminum tube and keeps insects from entering the home. (See also “screen”).

Rough Opening
- The opening in the wall where the window or door is to be installed, slightly larger than the size of the unit to allow squaring and shimming.

R-Value- Resistance to heat flow.  A high R-value indicates that a window has good heat insulating properties.


Safety Glass- A strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for storm doors and entry doors.

Sash- The part of a window that holds the glass can be opened and closed in an operating window.  Fixed windows may also have sash--the glass is set into the sash, then the sash set into the frame.

Sash Lock- The piece of window hardware  that secures the sash to the frame on a casement or awning window, or secures two sash at the check rail on a double-hung or sliding window when the window is closed.

Screen- The framed, mesh fiberglass or metal exterior piece on a window or door that keeps insects from entering the home.

Seat Board- A board applied to the bottom of a bow or bay window, providing support as well as a flat, finished surface.

Single Glazing- A single panel of glass with no insulating airspace.

Single Hung Window
- A window which looks like a double hung, but the top sash is stationary.  The bottom sash slides up and down.

Single Strength Glass- Glass with thickness 3/32” or 2.5 mm.  Also used in picture frames-common window glass.

Sill- The horizontal piece forming the bottom of the frame on a window or door.

Sill Nosing
- A piece of moulding to a window sill exterior that acts as a drip edge.

Simulated Divided Lite (SDL)- Windows with grilles adhered to the interior and exterior of the window glass, in a variety of decorative options, to simulate the look of True Divided Lite.

Slider- (Also known as a “Slide-by”) A window that slides horizontally.

Sliding Patio Doors- Door panels that slide giving fast, easy access to the outdoors.

Spacer-  The linear object  that separates and maintains the space between  the glass surfaces of insulating glass.  Can be metal or better insulating material. (See Warm Edge Spacer.)

Storm Window- An extra window on the outside to protect an existing window, but mainly to increase the thermal resistance of the window.

Stacked- When two or more window units are joined vertically.

Stationary- A non-operating sash, panel or unit.

Stiles- The vertical  pieces of a window or door sash panel.

Stool- Often called a window sill, the stool is a piece of interior trim that protrudes from the bottom of a window.  Stools are commonly seen in 19th century homes; plants and knick-knacks are often placed on window stools.

Strike Plate- The piece of locking hardware on a door frame which receives the latch or dead bolt.

Sweep- A piece of weather stripping along the bottom of the door that makes contact with the threshold to provide a tight seal against air and water infiltration.


Tempered Glass- Float glass that is heated and then cooled rapidly to make it several times stronger than regular annealed glass.  Standard in all doors, tempered glass is safer than annealed glass because when it is broken it falls into pebble like pieces without sharp edges.

Template- A pattern used to provide window dimensions for radius top, geometrical or special units.

Thermal Break- A material of high thermal resistance placed between two metal sash, or installed between adjoining metal framing of metal windows, in order to reduce thermal conduction from indoors to outdoors.

Threshold- The bottom area of the door frame that you step over when entering or exiting though a doorway.

Tilt Latch- Hardware on some double hung windows that allows the sash to be tilted or removed for cleaning.

Tinted Glass- Window glass that absorbs light and heat radiation.  Also reduces glare and brightness.  More common in commercial applications.  Most commonly bronze or gray color.

- A window stacked above another window or door.

Triple Pane
- Three panes of glass with two air spaces between.  Low-E glass can be used to greatly increase thermal properties.

Triple-Slide/By (Or Slider)
- A three-sash window featuring a stationary center sash and two flanking sash that open and close by sliding sideways.

True Divided Lite (TDL)- Individual lites of single glass or insulating glass, separated by muntin bars.


U-Factor (formerly known as “U-Valve”)-  U-Valve measures heat flow through a window or door.  Low U-factor means better insulating properties and better energy efficiency, especially in the winter.  U-factor is the inverse of R-value. (U=1/R).

uPVC- uPVC refers to the chemical composition of the vinyl used in quality vinyl windows and doors.  Poly Vinyl Chloride is unique among plastics.


Veneer- A thin sheath of wood usually applied over a stable type of wood, creating a beautiful finish.

V-Grooved Glass- A type of glass with grooves for decoration.


Warm Edge Spacer- Refers to the type of spacer material used to separate panes of glass in an insulated window unit.  If the material conducts less heat or cold than an aluminum spacer at the edge of the glass, it is said to be “warm edge.”

Weather Stripping- A strip of resilient material designed to seal the sash and frame members to reduce air and water infiltration.

Weephole- Small holes drilled along the bottom edge of storm windows or vinyl windows to permit moisture condensation or wind-driven rain to drain away from the sill to the outdoors.

Weep System- A special interior formation of the sill designed to channel water away from your home to prevent moisture build-up that leads to rotting.

Weight & Pulley Double Hung- Old windows that have vertical cavities to accommodate metal weights that counter balance the sash when opened.