Glossary of Window & Door Terms

Have you ever found yourself trying to explain a problem with your automobile to a mechanic and found it difficult because you didn’t know the correct terminology? Have you ever left the barber with a cut that wasn’t quite what you were expecting because of a communication failure? At Clevernest, we want our clients to feel confident that they understand the quotes we provide and the products they expect. This handy glossary of terms can help you describe to us exactly what is going wrong with your current windows or doors and help you clearly understand the products in the estimates we provide.



  • Air infiltration:
    Industry test that measures the amount of air leakage through a window or door (the lower the number the better).
  • Apron:
    Decorative trim positioned directly underneath a window stool and installed flush against the wall
  • Arch window:
    4 sided unit with a curve at the top
  • Argon gas:
    Colorless, odorless gas used in the air space of double pane Low-E glass to increase the insulating performance.
  • Art glass:
    Decorative glass in a variety of colors/shapes/patterns used to accent a window or door
  • Astragal:
    Center post of a double door attached to the fixed or inactive door panel
  • Awning window:
    Unit with hinged sash that swings outward from the bottom; allows ventilation at the bottom



  • Balancer:
    Counter-weight mechanism to assist raising or lowering of a double-hung or single hung sash
  • Bay window:
    Window consisting of three or more units that angle out beyond the wall; often configured with a large center unit and two flanking units
  • Bow window:
    Window consisting of three or more units projecting out from wall to form a radius
  • High Altitude Capillary Breather Tubes:
    Very small diameter tubes placed within the unit that allows equalization of the air space due to high elevation pressure differences.
  • Brick mould:
    Exterior trim around the window frame traditionally used to attached the window to the wall



  • Casement window:
    Unit with hinged sash that opens to the side; allows top to bottom ventilation
  • Casing:
    Flat, decorative molding used on the interior perimeter of a window or door that covers the space between the unit and rough opening or between units
  • Check rail:
    Located on double-hung windows where the bottom sash and top sash meet and the lock/keeper is mounted
  • Circle Top™ (Half Round) window:
    Half circle unit consisting of a curved top and linear bottom
  • Cladding:
    Low maintenance covering or coating attached to the unit exterior
  • Clear opening:
    The size of the opening created when a unit is in a full open position
  • Clerestory:
    Window located up high on wall; typically unreachable from ground level
  • Coil stock:
    Roll aluminum that is bent into shape to form a transition piece between the unit exterior and siding/trim
  • Combination unit:
    Storm window and insect screen contained in a single frame
  • Condensation:
    Water that collects as droplets on the glass/sash/panel interior or exterior under certain conditions (typically cold services when exposed to humidity).
  • Conversion kit:
    Retrofit kit to add tilting capability to 1968 and newer Andersen® Perma-Shield® Narroline® windows
  • Cornice:
    An ornamental molding at the top of the window positioned above the exterior trim
  • Cottage window:
    Double-hung window where the upper sash is shorter than the bottom sash



  • Damage Function:
    Percent of the Ultra-Violet (UV) and Visible Light energy from the sun that can cause fabric fading. The lower the number, the less potential for fabric fading. Preferred over just looking at UV transmission.
  • Daylight opening:
    Visible glass area
  • Decorative drip cap:
    An ornamental trim piece positioned at the top of the window above the exterior trim that directs water away from the top of the window
  • Design Pressure:
    Pressure (measured in pounds per square foot-psf) required by building codes to meet windload requirements. Our product “PG” -Performance Grade rating is used to determine compliance.
  • Direct glazed/ direct set:
    Glass is directly glazed into frame; stationary unit
  • Double-hung:
    Venting upper and lower sash in a single frame that slide vertically past one another
  • Dowel joint:
    A corner joint created by precisely boring matching holes into a door rail and style and joining them together with a dowel pin
  • Drip cap:
    One piece aluminum or vinyl cover installed above windows/doors that directs water away from the top of the unit
  • Dual pane/Double pane glass:
    Two panes of uncoated (non-Low-E) glass with an air-filled cavity (not argon-blend gas-filled)



  • Egress window:
    A venting window required by buiding codes for emergency escape and rescue, which are typcially required in bedrooms and which are required to meet certain minimum opening dimensions.
  • Electric operator:
    A motorized device that provides for remote operation of an awning window
    A government-backed program to help consumers identify energy efficient products.
  • Equal lite:
    Window or door with equal spaced grille bars
  • Escutcheon plate:
    Decorative door handle plate that conceals the locking mechanism
  • Extension jamb:
    Wood component fastened to the interior of the window/door that extends the window frame out to the wall depth
  • Exterior trim:
    A decorative trim positioned around the exterior perimeter of a window or door
  • Extruded aluminum:
    Aluminum that is shaped by running it through a dye, typically more durable than roll-formed material



  • Fenestration:
    Refers to an opening in a structure such as windows, doors and skylights. Can also refer to the placement of windows and doors in a building
  • Fibrex®:
    Andersen brand name for a durable PVC and wood fiber composite material used in windows and doors
  • Film:
    Removable, semi-transparent material that adheres to and helps protect the glass during construction
  • Fixed/Stationary:
    Non-venting or non-operable
  • Flashing:
    Water resistant material that directs water to a drainage plane or to the exterior surface of a building.
  • Flexiframe®:
    Andersen brand name for fixed geometric windows shapes (octagon, pentagon, trapezoid)
  • Folding door:
    Door unit with multiple hinging panels that can be folded together to create an large, unobstructed opening
  • Foot lock:
    Auxiliary lock used on gliding doors to secure the operating panel to the sill
  • Frame:
    Outer structure of a window or door that holds the sash or panel in position
  • French casement window:
    Unit with two venting sash that open outward to provide a large center opening with no center post
  • French door:
    Hinged door(s) with large glass area surrounded by a wide wood side stiles and a tall bottom rail
  • Full divided light:
    Grille intended to replicate the look of a True Divided Light unit; consists of an interior grille, exterior grille and spacer between the glass panes
  • Full frame:
    Frame intended for installation direct to the rough opening; opposite of insert window



  • Glazing: Glass in a window sash or door panel; the act of installing glass in a window sash or door panel
  • Glazing bead: Wood or vinyl pieces around the perimeter of the glass that covers the space between the glass edge and sash/panel
  • Gliding door/sliding door: Door with two or more panels where one panel slides horizontally past another
  • Gliding window/sliding window: Window with two sash, where one sash slides horizontally past the other
  • Grilles/Muntins/Dividers/Bars/Grids: Components used to simulate individual pieces of glass within a sash/panel
  • Grilles-Between-the-Glass (GBG’s)/Finelight™: Grille bars that are placed between the glass panes; allows for easy cleaning
  • Gusset plate: Metal plate attached to a window or door to strengthen a window or door joint



  • Half insect screen: Insect screen that covers the opening area of the operating sash in hung and gliding windows
  • Handing: Opening direction of hinging windows and doors; direction is determined by location of hinge on unit as viewed from exterior
  • Head board/Seat board: Stain grade wood shaped to the angles of a bay or bow window that form the finished upper and lower interior surfaces
  • Header: Heavy beam extended across the top of the rough opening to prevent the weight of the wall or roof from resting on the window frame



  • Impact Resistant glass: Laminated glass frequently used in hurricane-prone areas for protection from wind-borne debris
  • Inactive panel/sash: Panel/sash that will open only after active panel/sash is opened
  • Insect screen: A tightly woven mesh attached to a frame; allows outside air ventilation while keeping insects out
  • Insert window: A new window unit intended to be installed inside the frame pocket of an existing window
  • Insulating glass unit: Two or more glass panes that are sealed together to increase energy efficiency



  • Jalousie window: Louvered window with multiple horizontal glass sections that rotate open or closed
  • Jamb: Window or door frame members that form the top and sides of a unit
  • Jamb clips: Metal brackets used to secure a unit to the rough opening from the interior
  • Jamb liner: Interior frame cover surface where the sash slide on double-hung window
  • J-channel: Receiver components that surrounds a window intended to hide the seam between the window and siding



  • Keeper: Receiver on a sash that the lock engages
  • Keyed lock: Patio door lock that enables locking and unlocking from the home’s exterior



  • Laminated glass: Layered glass that resists breakage and holds together when broken
  • Laminated Veneer Lumber (LVL): Engineered wood consisting of multiple layers of thin wood bonded together
  • Lift: Handle on the bottom of a double hung window that makes it easier to raise/lower the sash
  • Light/Lite: Individual glass panes within a window sash or door panel
  • Lock: Mechanism used to secure windows and doors into a closed position
  • Low-E glass: Glass with a low-emissivity coating that restricts heat loss



  • Masonry opening: Opening in a brick, stone or stucco wall where a window or door is installed
  • Measurement guide: Step-by-step tool to help in determining the dimensions of replacement windows
  • Meeting stile: Central location on a gliding window or door where the sash/panel overlap
  • Monolithic glass: Glass that has no air space; used in specific situations such as some coastal applications
  • Mortise-and-tenon: Strong wood joint made by fitting together a slot (mortise) in one board and the matching projecting member (tenon) on the adjoining board
  • Mulling: Joining of two or more window or door units together; joint can run vertically, horizontally or both
  • Mullion: The vertical or horizontal joint between individual window or door units that form a combination
  • Multi-point lock: Lock that engages the sash or panel in multiple locations; activated by a single motion



  • Nailing flange / Installation flange: Narrow attachment strip on the window perimeter that typically used to secure the window to the rough opening
  • NFRC: National Fenestration Rating Council is an non-profit organization that provides for fair, accurate and credible energy performance ratings for windows, doors & skylights.



  • Obscure glass: Glass with a texture of pattern of various degrees of opacity that limits visibility through a window or door (see also patterned glass)
  • Operator: Metal arm, gear and handle used to open and close hinged windows
  • Outdoor/Indoor Transmission Class (OITC): Measurement standard used to indicate the rate of sound transmission between indoors and outdoors (see also STC)
  • OX / XO: “OX” or XO” are used to identify the opening direction of gliding windows/doors; “X” stands for operating while “O” stands for stationary



  • Palladian window: Large, arch-top window flanked by smaller windows on each side
  • Panel: Refers to the rail, stile and glass assembly on a door; similar to a window sash
  • Panning kerf: Narrow groove on the exterior outer frame edge of a unit typically used to accept trim or aluminum coil stock
  • Parting stop: A narrow frame molding that holds a sash or panel in position
  • PassiveSun™ glass: Andersen brand name for a glass type ideal for situations where solar heat gain is desired, typically in passive solar building designs.
  • Patterned glass: Glass that lets in light while obscuring vision (see also Obscure glass)
  • Performance Grade (PG): An industry rating for a unit that has been tested for air, water, structural and force entry performance. Used to determine compliance with code required design pressures.
  • Perma-Shield®: Andersen brand name for low-maintenance exterior cladding
  • Plinth block: Decorative wood block typically found at the top corners of the unit that creates a transition from vertical to horizontal interior trim
  • Pocket door: A door panel that can be concealed by a wall; panels slide into wall
  • Pre-finished: Factory painted or stained unit that is ready for installation; no finish coat required
  • Pultrusion: Strong, stiff composite material, in profile form, made from polymeric resin and glass fiber. Also used to refer to the process for producing these profiles
  • Push-out casement: A casement with no crank handle; window is opened by releasing sash lock and pushing the sash outward



  • Rail: Horizontal components of a window sash or door panel framework
  • Reinforced mull: Mull that uses a structural material (wood, aluminum, steel, LVL) to provide a stronger joint
  • Removable Interior grille: Interior grille that can be readily removed for cleaning
  • Reverse Cottage/Oriel window: Double-hung window where the upper sash is larger than the bottom sash
  • Rough opening: Opening in a wall for the installation of a window or door. The rough opening is larger than the actual unit to allow for shimming and insulating.



  • Sash: Rail, stile and glass components joined together the form the venting capability of a window
  • Sash glazed /Sash set: Fixed window with a separate sash and frame intended to replicate the look of nearby venting windows
  • Screen: A tightly woven mesh attached to a frame; allows outside air ventilation while keeping insects out
  • Seal: A compressible surface that inhibits air and water passage
  • Segmented arch: Multiple arch windows mulled horizontally to provide a single continuous arch
  • Shim: A wedge often made of wood used to plumb and/or level a window or door in the rough opening
  • Sidelight: Tall, narrow unit placed alongside a window or door
  • Sill: Horizontal member that forms the bottom of a window or door frame
  • Sill Nose: Traditional exterior trim component that attaches underneath and extends out beyond the sill
  • Simulated divided light: Use of interior and exterior grille with no spacer between the glass panes; used to simulate the look of a window with multiple glass lites
  • Single-hung: Double-hung styled window in which the top sash is inoperable
  • SmartSun™ glass: Andersen brand of a glass option that reduces solar heating but allows high visible light transmission
  • Solar Heat Gain Coefficient (SHGC): A measurement of the sun’s heat energy that passes through the window/door. The lower the value the less heat gain thru the product.
  • Sound Transmission Class (STC): Rating system used to measure how much sound is transferred through windows/doors (see also OITC)
  • Springline™ windows: Round top picture windows
  • Stile: The vertical components of a window sash or door panel
  • Stool: Flat, interior trim member located at the sill of a window or door, typically extending into the room and horizontally beyond the extension jambs
  • Stop: A trim member attached to the window or door intended to hold, position or separate components
  • Stormwatch® protection: Andersen brand of window and door package that improves product performance and satisfies coastal code requirements
  • Sun glass: Tinted glass that reduces heat gain
  • Sustainable Forest Initiative (SFI): Forest certification standard dedicated to promoting sustainable forest management



  • Tempered glass: Safety glass that is stronger than annealed glass and breaks into small pieces
  • Threshold: Decorative wood component used as a transition door sill to the interior flooring
  • Top hung insect screen: Insect screen used on patio door units that are secured from the top; results in smoother operation
  • Transition block: A decorative wood piece used to cover the joint between extension jambs or casings
  • Transom: Window positioned directly above another window/door
  • Triple pane: Glass construction consisting of three distinct layers of glass and two air-spaces
  • True Divided Light (TDL): Windows and doors in which multiple smaller glass lites are framed in a single sash or panel
  • TruScene® insect screens: Andersen brand of a patented micro-fine stainless steel mesh that provides 50% more clarity than typical Andersen insect screen cloth



  • U.S. Green Building Council (USGBC): Non-profit trade organization that promotes sustainability in how buildings are designed, built and operated
  • U-Factor: Commonly used measurement of heat transmission through a window/door. The lower the U-Factor, the better the insulating value.
  • Unobstructed glass opening: Visible glass area
  • UV protection: Percentage of Ultra-Violet (UV) transmission from the sun’s energy. The lower the number, the potential for reduced fabric fading. (See Damage Function)



  • Vapor barrier: Watertight material used to prevent moisture migration around windows and doors
  • Vent Limiter: Device that restricts window operation and cannot be disengaged without tools or special effort.
  • Venting unit: Window or door that opens; operates
  • Vinyl: Polyvinyl chloride (PVC) material that is very durable and resistant to corrosion.
  • Visible light transmission: The fraction of the light allowed to enter through a window



  • Weatherstrip: Compressible material designed to seal the sash or panel to the frame
  • Weep holes: Small holes placed on the exterior of a window or door that allows for water drainage
  • Window Opening Control Device (WOCD): Device intended to restrict the opening dimension of a window when first activated to less than 4 inches, but can be disengaged to allow full opening of the sash for escape and rescue, but reengages when the sash is closed such that the sash will open to a less than 4 inch dimension when opened again.