Below is a diagram of a popular window different parts highlighted. Other types of windows may have slightly different components.

 
  1. Head – The main horizontal member which forms the top of the window frame.
  2. Grids – Exterior and interior grid options are available with some windows to better simulate real divided lites.
  3. Hardware – Window locks and operating handles.
  4. Weather-stripping – Weather-stripping is essential to ensure efficient, weather tight seals for your windows.
  5. Insulated Glass – A combination of two or more panes of glass with a hermetically sealed air space between them. 
  6. Upper Sash – A single assembly of stiles (sides) and rails (top and bottom) made into a frame for holding insulated glass. The top sash is known as the Upper sash.
  7. Jamb – The window frame sections forming the top, sides and bottom of a window frame.
  8. Lower Sash – A single assembly of stiles (sides) and rails (top and bottom) made into a frame for holding insulated glass. The bottom sash is known as the Lower sash.
  9. Sill – The horizontal section (ledge) forming the bottom of the window frame.

Window Glossary and Terms


Air infiltration — the flow of air that passes through windows and doors. On NFRC labels, the lower the number, the better.

Airspace — the space between layers of glass, often filled with argon or krypton gas.

Apron — horizontal trim board under a window stool.

Awning— a window consisting of one sash hinged at the top which projects outward from the home.

Backband — additional trim piece on the outside edge of window casing, usually installed when the casing is flat boards.

Balance — a mechanical device used in hung windows as a means of counterbalancing the weight of the sash. Common balance systems are constant force, block and tackle, and spiral.

Bay window — window that projects out from the wall, usually consisting of 3 window units with the largest unit in the center.

Bottom rail — bottom horizontal member of a window sash.

Bow window (also compass, radial bay window) — rounded bay window that projects from a wall in the shape of an arc. It consists usually of five units.

Brickmould — a moulding used as an exterior door or window casing.

British thermal unit (Btu) — the heat required to increase the temperature of 1 lb. of water 1°F.

Caming — Formed metal stripping, usually made of brass or zinc plated steel, used between cut-glass pieces to assemble the pieces into a decorative glass panel. Caming is soldered at joints to bond the glass assembly together.

Casement window — a window with one sash hinged to open from the side and project outward from the home

Casing — a trim used to cover the space between the window frame or jamb and the wall.

Chemically bonded (when related to a welded corner) — a process where the two polymer profiles or pieces are heated and fused together with the aid of a chemical reaction. The reaction and bonding is similar to the original extrusion process.

Cladding — material that covers the frame, sash, or sliding door panel exteriors and make a weather-resistant surface.

Condensation — deposits of moisture (water droplets or frost) on the surface of an object caused by warm, moist air coming into contact with a colder surface.

Corrosion — deterioration caused by chemical or electrochemical reactions resulting from exposure to weather, moisture, chemicals, or other harmful materials.

Design pressure (DP) — a rating that identifies the wind load that a product is rated to withstand in its end-use application, measured in pounds per square foot (psf).
Note: Design pressure (DP) is not to be confused with Performance Grade (PG) or structural test pressure (STP).

Divider — a member that divides the glass area of a window into separate areas. Dividers are either structural or decorative. Other common terms are muntin, true divided lite (TDL), simulated divided lite (SDL), grill, grid, or bar-in-glass.

Dormer window — window in a wall that either projects from a sloping roof, or is recessed (inset dormer) into the roof, or a combination of both.

Double-hung window —window with two sashes that can move vertically past each other to allow venting.

Drip cap — horizontal exterior molding to direct water from the top of the window or door unit so water drips beyond the outside of the frame.

Dual pane (Dual glazing) — two layers of glass separated by a spacer and airspace, and sealed.

Egress — window that is required in sleeping locations in a home and is intended to provide emergency escape and rescue access. Windows must meet specific size and requirements to qualify as an egress window. Requirements are defined by local building codes.

ENERGY STAR® — a U.S. Environmental Protection Agency (EPA) voluntary program that helps businesses and individuals save money and protect our climate through superior energy efficiency.

Escutcheon plate: decorative plate behind a door handle that conceals the locking mechanism.

Extension jamb (also jamb lining and jamb extender) — a board used to increase the depth of the jambs of a window frame to fit a wall of any given thickness.

Fenestration — openings in the building envelope, such as windows, doors, secondary storm products (SSPs) curtain walls, storefronts, roof windows, tubular daylighting devices (TDDs), sloped glazing, and skylights, designed to permit the passage of air, light, or people.

Fixed window — a window that is designed to be non-operable and consists of a glazed frame or a non-operating sash within a frame.

Flashing —a strip of water-resistant material that diverts water away from a window or door.

Forced-entry resistance (FER) — the ability of a window or door in the locked position to resist entry under a specified load and conditions.

Frame — the enclosing structure of a window or door, which fits into or attaches to the wall opening.

Full frame — type of installation which requires the frame of the old window to be completely removed, allowing the new window to be installed into the studded wall opening.

Fusion welded — when materials are fused by heating the edge and pressing the two pieces together. When cooled, they become one piece.

Garden window — a window consisting of a three-dimensional, five-sided structure, often with shelves for plants in the enclosed space that protrudes outside the home.

Glazing — (n): glass in a window sash or door panel. (v): the process of installing glass in windows and doors

Glazing bead (also glass stop and wood stop and sill bead) — removable trim that holds glass in place.

Handing — opening direction of hinged windows and doors; direction is determined by location of hinge on unit as viewed from exterior

Handle — a component which enables the movement of a sash or panel, or which activates a mechanism which locks or unlocks a sash or panel.

Hardware — all the necessary equipment to retain, operate, and lock or unlock the sash or panel within the frame.

Head — the horizontal member forming the top of a window or door.

Header (also lintel; beam) — horizontal member (wood, steel, or stone) over a window opening to support the weight of the wall above. The term header is generally in reference to a wood beam, whereas “Lintel” often refers to a steel beam.

Hopper window —a window consisting of one sash hinged at the bottom which opens inward from the frame.

Insulating glass unit (IG unit or IGU) — two or more panes of glass spaced apart with a perimeter spacer and hermetically sealed to form a single unit with an air- or gas-filled space between each pane.

Jambs — the upright or vertical members forming the sides of the frame.

Jamb depth — width of the window or door frame from inside to outside.

J-Channel: installed or built-in to the side of a window or door, this channel is designed to accommodate the ends of siding pieces to provide a finished appearance.

Keeper — hardware on a sash that receives and engages with lock

Laminated glass — two or more panes of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for enhanced safety and security, as well as sound reduction.

Lite — a pane of glass or an insulating glass (IG) unit used in a window or door.

Low-emissivity (Low-E) glass: glass with a coating that reflects heat.

LRRP — the lead paint Renovation, Repair and Painting program established by the Environmental Protection Agency for pre-1978 homes to address health and safety issues associated with lead paint. Firms involved in such projects must be trained and certified in lead-safe work procedures. Also referred to as RRP. Information available on EPA Web site.

Meeting rail or check rail — one of the two adjacent horizontal sash members that come together when in the closed position.

Mulling — the process of joining two window or door units together

Mullion — component used to structurally join two window or door units. The joint can run vertically or horizontally.

Multi-point lock — a lock operated with one handle that engages the sash or panel in multiple locations

Muntin — see Divider.

Nailing fin — An accessory component or integral extension of a window or patio door frame that generally overlaps the conventional stud construction. Nails can be driven through it to secure the frame in place.

Negative pressure — pressure acting in the outward direction.

NFRC — National Fenestration Rating Council. A body that has established methods for rating and certifying the energy performance of windows, doors, skylights and other fenestration products.

Obscure glass — see Privacy Glass

Operator — hardware designated to open and close hinged windows and blinds.

Outdoor-indoor transmission class (OITC) — a single-number rating that provides an estimate of the sound insulation performance of windows and doors. Note: The frequency range used is typical of outdoor traffic noises.

Panel — the members of a sliding door or sliding door side lite within a frame which are designed to accommodate the glazing.

Parting bead (also parting strip and parting stop) — a vertical strip on each jamb that separates the sashes of a double-hung window and holds the sash in place.

Performance Grade (Grade or PG) — an industry rating for a unit that has been tested for air, water, structural and forced entry performance. Often a factor of compliance with local building codes.
Note: Performance Grade (Grade or PG) is not to be confused with design pressure (DP) or structural test pressure (STP). Performance Grade (PG) is achieved only on successful completion of all applicable tests specified

Picture window — large fixed windows.

Privacy Glass — glass with an irregular surface for scattering light; used for privacy or to reduce glare.

Pocket window — type of installation which the sashes and jamb tracks are removed, and a new window installed within the old frame.

Positive pressure — pressure acting in the inward direction.

Projected Frame window — window with a frame with extended jambs and projects outward from the home.

Rail — a horizontal member of a sash or panel.

Reinforcement — the material added to individual sash, panel, or frame members to increase strength and/or rigidity.

Rough opening — a structurally-framed opening in a wall which receives a door unit or window.

R-value: numerical rating designating the resistance to thermal transfer or heat flow. Higher R-value numbers indicate greater insulating value. R-value is frequently used by the insulation industry and is the reciprocal of U-factor, a value more generally used in the window industry.

Safety glass — a strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for doors and some windows. See also Tempered glass and Laminated glass.

Sash — an assembly of stiles and rails (vertical and horizontal members) made into a frame for holding glass.

Screen — a product that is used with a window or door consists of a mesh of wire or fiberglass attached to a frame to keep out insects. It is not meant for providing security or retention of objects or persons from the interior.

Seal — material around the edge of an insulated glass unit that protects the glass unit from air and water leakage.

Sidelite — an operable or non-operable product that is designed to be a companion product installed on one or both sides of an operable door or a fixed door.

Sill (also sill plate; inside sill; outside sill) — horizontal member at the bottom of the window or door frame

Simulated divided lites (SDLs): A type of grille or grid design that creates the appearance of a number of smaller panes of glass separated by muntins, but actually uses larger lites of glass with the muntins placed between and/or on the surfaces of the glass layers.

Single glazing — glazing that is just one layer of glass or other glazing material.

Single-hung window — a hung window with only one operable sash.

Slider — a window that consists of one or more sashes that slide or roll horizontally within a common frame and can also contain fixed lites/sash.

Sliding door — a door that consists of manually operated door panels, one or more of which slide or roll horizontally within a common frame, and can also contain fixed lites/panels.

Solar heat gain coefficient (SHGC) — a rating measuring a window’s ability to transmit solar heat. It measures both the solar radiation which is directly transmitted, as well as the solar radiation absorbed by the glass and subsequently transmitted. The lower a unit’s solar heat gain coefficient, the less solar heat it transmits. It is expressed as a number between 0 and 1.

Sound transmission class (STC) — a rating measuring a window’s acoustic properties or its ability to reduce sound transmission. An STC rating is determined by measuring the sound transmission over a selected range of sound frequencies. The higher the number, the less sound transmitted.
Note: The frequency range used is typical of indoor office noises.

Spacer — the linear material that separates and maintains the space between the glass surfaces of insulating glass units.

Stile — a vertical member of a sash, leaf, or panel.

Stool— a horizontal board or trim piece at the windowsill on the inside of the window, fitted against the bottom rail of the lower sash to form a base for the casing.

Storm window — that window in a dual-window unit so designated by the manufacturer, used on the exterior of, or interior of, and in tandem with, a primary window for the purpose of performance enhancement, and not to be used by itself as a primary window.

Structural test pressure (STP) — the pressure differential applied to a window or door system.
Note: Structural test pressure (STP) is not to be confused with design pressure (DP) or Performance Grade (PG).

Tempered glass — glass that has been heat-treated to make it strong. If broken, it will fracture into small pieces without sharp edges. Tempered glass is approximately four times stronger than annealed glass of the same thickness.

Thermal barrier — an element made of material with relatively low thermal conductivity, which is inserted between two members having high thermal conductivity, in order to reduce the heat transfer.

Transom — an operable or non-operable product that is designed to be a companion product installed above a window or door.

Triple pane — three panes of glass separated by two spacers and air-spaces.

U-factor — rate of heat flow-value through a building component, from room air to outside air. Also referred to as U-value. The lower the U-factor, the better the insulating value.

Weatherstrip (weatherseal) — a flexible component used to reduce air leakage, water penetration, or both between sash and/or frame.

Weephole (weep) — an opening that allows water to drain.