Glossary Of Terms

A List of Industry Related Terms Used By Window & Door Professionals

The glossary below is a comprehensive collection of terms used by professionals while discussing windows and doors. These terms range from the parts that make up a window, to performance unit measurements, industry related programs and a whole lot more. You can choose to scroll through the page to view all terms or click on an individual letter to view a list of terms starting with that letter.

Visit our Windows Basics section for more window and door learning resources.

A window with the jamb head and sill labeled

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z 

A - Terms

Air Chambers

Small honeycomb spaces within the sash and frame extrusions that help to insulate and strengthen the window.

Argon Gas

A gas that is heavier than air – it can be used to fill the airspace of an insulated glass unit. Argon is a safe, colorless, odorless, tasteless, non-toxic gas, which is six times denser than air. Argon increases the insulating value of an insulated glass unit as well as helps improve sound insulation qualities.

Awning Window

A window that is hinged at the top horizontal edge, and cranks open or shut from the bottom.


B - Terms


The system in the side jambs of a single or double-hung window that helps open and support the weight of the sash, and helps hold the sash in place while in an open position.

Bay Window

A bay window is generally made up of three windows. The side or flanker units project out from the building at 30, 45, or 90 degree angles. The center is parallel with the building wall and is made up of one or more joined windows. All of the units can be stationary, operating, or any combination of the two. Typically the center section is stationary, while the side units are operating for ventilation.

Beveled Exterior

An extension of the vinyl frame that adds an aesthetically pleasing dimension to the exterior of the window.

Bow Window

A series of three or more adjoining window units, commonly five in number, each connected at 10-20 degree angles to form a circular arch appearance.

Brick Mould

Exterior casing trim around a window or door. Brick mould covers the gap between the frame and masonry opening. In some cases, siding is installed up to the edge of the brick mould.

Buck Size

The exact (net) window size, not including the nail fin.


C - Terms

Casement Window

A window unit in which the single sash that opens outward to the left or right, projecting off the plane of the wall. The sash unit is hinged on one side and is operated by a crank mechanism.

Casement Window

A window unit in which the single sash that opens outward to the left or right, projecting off the plane of the wall. The sash unit is hinged on one side and is operated by a crank mechanism.


Inside casing is a flat, decorative moulding that covers the inside edge of the jambs and the rough openings between the window unit and the wall. Outside casing (or brickmould), serves the same purpose.


A compound for filing joints to prevent air and water leakage. Caulking is used where air and water leakage and/or movement may occur.

Circle Top

A generic term referring to any of a variety of window units having a curved top frame member, and are often used over another window or over a door opening.


The formation of water vapor from the air on any cold surface, whose temperature is below the dew point.

Cottage Window

A double hung or single hung window with a larger proportioned bottom sash, as compared to the top sash.


D - Terms

Design Pressure

The pressure a product is designed to withstand. This value is a measure of a product’s capacity to withstand the forces of wind loading, in both positive and negative directions, while it is closed and locked.

Dew Point

The temperature at which condensation occurs.

Divided Lite

A window opening divided into smaller sections by a grid system on the interior or exterior of the glass, between the glass panes, or any combination of these three.

Double Glazing

Two panes of glass separated by a sealed air space, forming a glass panel that increases energy efficiency and provides other performance benefits such as improved outside noise reduction.

Double Hung Window

A window unit that contains two vertically-sliding operable sashes, which move vertically in the frame.

Drip Cap

A molding designed to divert water from the top of a window unit so that water moves beyond the outside of the frame.


E - Terms


A government-backed program helping businesses and individuals protect the environment through the use of energy efficient products. ENERGY STAR® qualifying products, such as windows and doors, mean these items use less energy, save money and help protect the environment. Energy Star is a registered trademark of the U.S. Department of Energy.

Egress Window

A window opening large enough, as defined by local building codes, for exit or entry in case of an emergency. Typically required in bedrooms where no other means of exterior escape exists.


A particular cross-sectional shape produced by forcing material through a die.


F - Terms


An industry term of Latin origin that refers to the broader category of windows, doors and skylights.


A window that is non-venting or non-operable, such as a picture window.


A metal or plastic strip attached to the outside of the head or side jambs of windows and doors to provide a weather barrier, resisting leakage between the window or door frame and the wall.


The combination of head, jambs, and sill to form a precise opening in which a window sash or door panel fits.

Fusion Welded

Refers to vinyl frames and sash attached together at corner joints, using a heat source to create a fused, weather tight corner joinery.


G - Terms

Garden Window

A window that projects out from an exterior wall, often used as a greenhouse window for house plants. It has a slanted glass roof that allows heat and light from the sun to enter.

Geometric Window

Specialty windows of various shapes including: rectangles, triangles, trapezoids, octagons, pentagons, half-rounds, quarter rounds, full rounds sectors and ellipses.


The process of sealing glass to a sash or frame for a weather tight seal.

Glazing Bead

A removable decorative trim around the glass perimeter, covering the gap between glass and frame.


A decorative grid on the interior or exterior surface of the glass, or, more commonly found between sealed between glass panes in an insulated glass unit, or in any combination of these locations that divides a window opening into smaller openings to create simulated divided lite or true divided lite. Grilles may or may not be removable.


H - Terms


The main horizontal member forming the top of the window or door frame.

Hopper Window

The sash is hinged at the bottom so that the window opens into the house. Primarily used in basement applications.

Horizontal Sliding Window

One or more sash that slide horizontally past each other. One or more sash may be fixed (inoperable), or each sash may operate to open and close.


I - Terms


Air that is able to flow through cracks and other spaces around a window or door, and also at the meeting rail within a window unit.

Insulated Glass (IG)

A combination of two or more panes of glass with an hermetically sealed air space between the glass panes. This space may or may not be filled with an inert gas, such as argon gas.


J - Terms


“J” configuration designed into window frame exterior shape for the primary purpose of receiving siding to self-trim around a window perimeter.


A series of rectangular, horizontal overlapping glass slats held together by metal end-frames attached to a side-jamb mechanical unit that simultaneously opens or closes the slats in unison.


The main vertical members forming the sides of a window or door frame.


K - Terms

There are no terms that start with this particular letter.


L - Terms

Lift Rail Handle

Handle or grip installed on the sash of a window to make it easier to raise or lower the sash.


A unit of flat glass; one glass panel expressed as a “lite”.

Low-E (Emissivity) Glass

Glass treated with a thin transparent coating of metallic oxide (generally silver). Allows natural light and short-wave heat energy to freely penetrate glass during the winter, while reflecting long-wave heat energy back outside during the summer months. Helps keep your home cool in the summer and warm in the winter. It greatly reduces ultraviolet light to enter the home, minimizing fading exposure to carpet and furnishings.


M - Terms

Meeting Rail

The joinery point where horizontal or vertical sections of the top and bottom sash meet when the window is closed. On sliders, the meeting rail is vertical. On single and double hung windows, the meeting rail is horizontal.


Strips of wood usually shaped to a curved profile, used to accent and emphasize the ornamentation of a structure and to conceal surface or angle joints.

Mulled Unit

Two or more window units structurally joined together.


A metal or vinyl extrusion used to structurally join two or more windows.


The pieces of decorative grid that help divide a window opening into smaller sections. Also called a grille or a grid.

Muntin Bar

See: muntin.


N - Terms

Nailing Fin

Used to secure the window into a rough opening.


O - Terms

Obscure/Translucent Glass

A frosted or textured glass that transmits light, but obscures the view.

Oriel Window

A double hung or single hung window with a larger proportioned top sash, as compared to the bottom sash.


P - Terms


A framed sheet of glass within a window or door frame.


Usually refers to the separate panel or panels in a door frame. A panel may be operable or stationary (fixed).

Picture Window

A fixed window that contains no operable sash.


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: a 4/12 pitch means the roof rises four inches for every running foot of horizontal span.


Q - Terms


A group of windows mulled in combination of fours.


R - Terms


Resistance to thermal transfer or heat flow. Higher R-value numbers indicate greater insulating value. It is the inverse of the U-Value (R=1/U).


The horizontal top and bottom members of a window sash or door panel.

Replacement Window

A window designed to replace and fit into an existing window opening once the old window is removed.

Rough Opening

The framed opening in a wall into which a window or door unit is to be installed.


S - Terms


A single assembly of stiles and rails made into a frame, designed to hold the glass in a window, which is then set into a main frame. A sash may be operable or inoperable.

Sash Balance

A system of cords, and/or springs that assist in raising a sash, while keeping the sash in any placed position by counter-balancing the weight of the sash.

Sash Lock

Generally, a cam-action or other latch-type lock applied to the sashes of a sliding window to both pull the sashes tightly together and to seal the sash tightly to the frame, both for security and to create a weather tight seal.

Sash Stop

An extrusion molding piece, generally about 2” long that covers the joint between window sash and the jamb, stopping the operable sash at its maximum opening.


Wood, plastic or composite wedges used to secure the window or door unit in the rough or masonry opening in a square, level and plumb position both during and after installation.


Tall, narrow, fixed or operating sash on either or both sides of a door to light an entryway or vestibule.


The main horizontal member forming the bottom frame portion of a window or door.

Simulated Divided Light (SDL)

A method of constructing windows in which muntins are affixed to the inside and outside of a panel of insulating glass to simulate the look of true divided light panes.

Single Glazing

Use of a single pane of glass in a window. Not as energy-efficient as insulated glass.

Single Hung Window

Window with a fixed top sash and a vertically operating bottom sash.

Slider Window

Window that slides horizontally to the left or right.

Sliding Patio Door

A patio door in which a vent panel moves horizontally on a sill track system past a fixed or operable panel.

Solar Heat Gain Coefficient (SHGC)

A measure of how effective a window or door is at keeping out solar heat. It is calculated by taking the amount of solar heat that enters a room through a window or door, divided by the amount that is actually contacting the exterior of the unit. The lower the value, the better the unit keeps out solar heat.

Sound Transmission Class (STC) Rating

Measures the amount of noise reduction that can be achieved with a given product. The higher the number, the better the product is at suppressing sound transmission.


The vertical member of a window sash or frame, or of a door panel.


T - Terms

Tempered Glass

A glass panel that is heated and subsequently rapidly cooled in its manufacturing process, creating a product that can withstand abnormal force or pressure on its surface, and which does not break into sharp pieces (also known as ‘safety glass’); code requires tempered glass in all doors (including patio doors), and in windows that are located near doors, bathtubs, or showers.

Thermal Break

An element of low conductance placed between elements of higher conductance to reduce the flow of heat (or cold). In thermal break window frames, a non-metallic polyurethane material separates the transmission of cold between the frame’s exterior and the frame’s interior.

Tilt Window

A window designed in such a way that the sashes tilt inward for easy cleaning of the outside glass surface.


A small window placed over the top of a door or window, primarily for additional light and aesthetic value.


A group of windows mulled (joined) in combination of threes.


A group of windows mulled (joined) in combinations of twos.


U - Terms

U-Value (U-Factor)

A calculation expressing the rate of heat transfer through a window or door. The lower the U-value, the better the insulating properties of the window or door.


V - Terms

There are no terms that start with this particular letter.


W - Terms


Various shaped metal, vinyl, plastic or fiber strips that fit tightly against the window or door frame to resist air and water infiltration through gaps and cracks.

Weep Hole

A small hole (or holes) found along the bottom sill frame edge of a window or door unit that allows any trapped water to escape to the exterior.

Wind Load

Force extended on a surface by moving air.


X - Terms

There are no terms that start with this particular letter.


Y - Terms

There are no terms that start with this particular letter.


Z - Terms

There are no terms that start with this particular letter.


Window Basics & Learning Resources

Learn More

Learn About The Energy Star Program

Learn More

Atrium Windows Buying Guide

Learn More

Windows 101 Video Series

Enjoy our collection of animated educational videos covering a variety of topics including condensation, window styles, window options, warranties, window/building codes and more.

Browse Windows 101 Videos