Below is a listing of common window and door terminology used in the fenestration industry:


In window and door terminology, absorptance is the ratio of radiant energy absorbed to total incident radiant energy in a glazing system.


A thermoplastic with good weather resistance, shatter resistance, and optical clarity, used for glazing.

Air Infiltration

Air leakage is measured by physical testing of a standard sized window. The test window is installed on a large wall. On one side, a vacuum is of 75 pascals (1.57 psf) is applied. This corresponds to approximately a 25 mph wind blowing perpendicularly to the window. Flow meters measure the rate of air leakage. This is divided by the total window area to get a reading (in the U.S.) in cfm/ft2 to the nearest tenth (i.e. a test value of 0.17 would be given a rating of 0.2). The best possible rating by the NFRC is 0.1, as they will not round to 0.

Aluminum Clad

Window and door terminology: A window or door made from wood with an exterior skin of aluminum.

Aluminum Window

A window whose frame and sashes are made from aluminum.

Argon gas

Argon is a safe, odorless, colorless, non-toxic, non-flammable, inert gas that is commonly used in place of air between the glass panes of an insulated Low-E glass unit.

Awning Window

A window unit which projects outward toward the exterior of the building as it opens. Hinges at the top of the side frame members, permitting the sash to open up and out like an awning.

Basic Unit

This term is given to the fundamental “building block” units of each standard window product line. These units are members of a series of standard sizes. They lack exterior trim or installation flanges but are otherwise complete.


Cosmetic covering, usually found on the exterior of the window or door to achieve aesthetic sight lines or to integrate the window or door system into the building surface or weatherproofing system. If panning is being used for weather, the panning is not considered cosmetic, but part of the window system.


A window sash that swings open on side hinges; in-swinging are French in origin; out-swinging are from England.

Casement Window

A projecting window with a single sash hinged at the sides and usually opening outward like a door and operated by a (crank) handle which turns to open or close the unit.

Casing (Trim)

Exposed molding or framing around a window or door, on either the inside or outside, to cover the space between the window frame or jamb and the wall.


A designation given to products whose exposed exterior surfaces are sheathed with specially formed aluminum.


Protective material, typically vinyl or aluminum, sometimes applied to the exterior of wood windows to prevent decay and reduce the need for painting.

Divided Lights

individual panes of glass separated by muntins within a window sash.

Door Jamb

The part of a door frame which surrounds and contacts the edges of the stiles and top rail of a door; jambs may be classified as (1) “head or “side” jambs and (2) “plain” or “rabbeted”.

Double Glazed Units

Units consisting of two lites of glass and one air space in between.

Double Glazing

In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.

Double Hung Window

Two sashes which move vertically, bypassing each other in a single frame. Sash may be counter-balanced by weights or springs.


ENERGY STAR is an independent U.S. government program establishing a standard set of guidelines to recognize the energy efficiency of various products. ENERGY STAR guidelines are used in conjunction with a variety of building materials, including windows and patio doors. Over the past ten years, ENERGY STAR guidelines have helped double the efficiency of windows they endorse.


The arrangement, proportioning, and design of windows and doors in a building; a common term to describe the window and door industry.

Fixed window, lite, or panel

Fixed means that it does not open or move.


The structural parts of a window, including the jambs and reinforcing members.

Gas fill

Heavier-than-air gas, such as argon or krypton, sometimes used to fill the air space in windows with insulating glass to boost energy performance.


Glass; used to refer to the type of glass or system used in a particular window, such as, plate glass, tempered glass, Low-E, or tinted.


One-piece decorative latticework that may snap into a sash to simulate divided lights.

Hung Window

A window in which the operating sash move up and down within the master frame. The weight of each operating sash is counterbalanced with balances to permit easy operation.

I.G. Unit (Insulating Glass Unit)

Two or more lites of glass separated by a spacer and hermetically sealed at the glass edges.

Insulating glass

Two or three panes of glass separated by an air space.


The sides of a window assembly.


An inert, odorless, colorless, tasteless, nontoxic gas which is about 12 times denser than air. It is used to replace air between the glass panes to reduce temperature transfer and deter convection. The gas provides a higher performance than windows produced without it.


A composite unit of glass and inter-layer material.

Laminated Glass

Two or more pieces of glass bonded together over a plastic inter-layer.


A unit of glass, in window or door that is enclosed by the sash or muntins and bars. This can also be called a pane.

Low-E Glazing

Glass or plastic treated with a transparent metallic film to improve resistance to thermal transfer.


Narrow strips that separate individual panes of glass in a window sash.


A major component of a sliding glass door, consisting of a lite of glass in a frame installed within the main (or outer) frame of the door. A panel may be sliding or fixed.

Picture Window

Non-venting or non-operable window. Also known as a fixed window.

Pivot Window

A window with a sash that swings open or shut by revolving on pivots at either side of the sash or at top and bottom.

Prime Window

An entire window assembly including sash and jambs.

Rough Opening

Opening in a structural frame meant to accommodate a window.


Measurement of resistance to heat flow; usually refers to the rating of glazing—the higher the R-value, the better the window.

Safety Glass

A strengthened or reinforced glass that is less subject to breakage or splintering.


Window component that retains glass.

Sash Balance

Mechanism that controls the operation of double-hung window sashes.

Single Hung Window

A window consisting of two sashes of glass, the top one stationary and the bottom movable.

Skylight (operable or pivot)

A roof window that gives light and ventilation.

Sliding Window

A window fitted with one or more sashes opening by sliding horizontally or vertically in grooves provided by frame members. Vertical sliders may be single- or double-hung.

Solar Heat Gain Coefficient (SHGC)

The fraction of solar radiation admitted through a window or skylight, directly transmitted as well as absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window’s shading ability. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits, and the greater it’s shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.

Sound Transmission Class (STC)

The sound transmission loss rating of a material over a selected range of sound frequencies. The higher the number, the less sound transmitted.

Tempered Glass

Glass with a surface compression of no less than 10,000 psi, or an edge compression of no less than 9,700 psi. When broken, the glass breaks into pebbles instead of shards.

Tilt Window

A single- or double-hung window whose operable sash can be tilted into the room for interior wash ability.

Tinted Glass

Glass with a material added to give the glass a light and/or heat reducing capability and color.


Measurement of thermal transfers; usually refers to a window’s overall rating—the lower the U-value, the better the window.


A plastic material used for cladding or entire window units. Vinyl is a generic term for modified PVC (Polyvinyl Chloride).

Vinyl Window

A window whose frame and sashes are made from vinyl.

Vinyl-Clad Window

A window with exterior wood parts covered with extruded vinyl.

Visual Transmittance

Visible transmittance is the fraction of visible light that comes through the product. This is influenced by glass selection, as well as the amount of opening taken up by non-transparent components such as the frame and sash. The greater the VT, the better the potential for daylighting. Normally, a reduction in SHGC (Solar Heat Gain Coefficient) comes with a reduction in VT.