Useful Terms

Argon Gas

Argon (IPA:/ˈɑːgɒn/) is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). Argon is present in the Earth’s atmosphere at slightly less than 1%, making it the most common noble gas on Earth. Its full outer shell makes argon stable and resistant to bonding with other elements

Bay Window

A multi-panel window, with at least three panels set at different angles, to create a protrusion from the wall line.

Brick Molding

A milled wood trim piece covering the gap between the window frame and masonry, which can be rectilinear, curved or composite-curved.

Casement Window

A window with a hinged sash that swings in or out like a door. There are options of choosing either a side-hung, top-hung, or occasionally bottom-hung sash or a combination of these types.  Occasionally there are fixed panels on one or more sides of the sash.


(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.

Double Hung

The traditional style of window in the USA, and many other places that were formerly colonized by the UK, with two parts (sashes) that overlap slightly and slide up and down inside the frame. The two parts are not necessarily the same size. Nowadays, most new double-hung sash windows use spring balances to support the sashes, but traditionally, counterweights held in boxes either side of the window were used. These were and are attached to the sashes using pulleys of either braided cord or, later, purpose-made chain.


An exterior face of a building.

Emergency Exit Window / Egress Window

A window big enough and low enough so that occupants can escape from the designated safety opening in emergency. In the United States, exact specifications for emergency windows in bedrooms are given in many building codes.


The main exterior face of a building, sometimes distinguished from the other faces by elaboration of architectural or ornamental details.


The organization and design of windows in a building.

Finish Casing

(Finish Trim) – Interior trim boards around a window unit.

Fixed Light

(Fixed Sash) – Window which is non-operative (does not open).


Single, Double or triple glass panes.


Small bars that divide the glass area.


The top of the window frame.


A movable joint enabling a window to swing open.

Insulated Glass

Insulated Glazing Unit or Insulating Glass Unit (commonly referred to as IGU) is described as two or more lites of glass spaced apart and hermetically sealed to form a single glazed unit with an air space between each lite. As the name suggests, the most important function of and IGU is to improve the thermal performance of glass when used in architectural applications.

Insulating Glass

IGU made of glass is called insulated glass (which refers to heat insulation). A more technically correct term in fact is insulating glass; since the glass itself has no insulation properties. It is actually the air space between the glass layers (lites) that provides the insulation qualities.

The air space between the lites may be filled with air or an inert gas like argon orkrypton which would provide better insulating performance. Typically the spacer is filled with desiccant to prevent condensation and improve insulating performance. Less commonly, most of the air is removed, leaving a partial vacuum, which drastically reduces heat transfer through convection and conduction. This is called evacuated glazing.

Often the insulating quality is used in reference to heat flow where the gap between glazed sheets is the insulating medium.

In some situations the insulation is in reference to noise mitigation. In these circumstances a large gap improves the noise insulation quality or Sound transmission class.

Insulated glass may not be cut to size in the field like plate glass but must be manufactured to the proper size in a shop equipped with special equipment.


The vertical member of the window frame.

Krypton Gas

A chemical element with the symbol Kr and atomic number 36. A colorless, odorless, tasteless noble gas; krypton occurs in trace amounts in the atmosphere. It is isolated by fractionally distilling liquified air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes but it is known to form compounds with fluorine.


(Catch, Lock) – A device which holds a window shut, such as the latch at the meeting rail of a double-hung window or one mounted on the stile of casement windows, often referred to as a lock.


A horizontal structural element over an opening, which carries the weight of the wall above it.

Low-E Glass

Low-E coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazed surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from warm surfaces to cooler surfaces. Coating a glass surface with a low-emittance material reflects a significant amount of this radiant heat, thus lowering the total heat flow through the window. Low-E coatings are transparent to visible light, and opaque to infrared radiation. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.

Meeting Rail

The rail of a double-hung window sash designed to interlock with the adjacent rail.


Vertical member between window units.

Multi-Lite Grids

A window glazed with small panes of glass separated by wooden or lead “glazing bars”, or “muntins”, arranged in a decorative “glazing pattern” often dictated by the architectural style at use. Due to the historic unavailability of large panes of glass, this was the prevailing style of window until the beginning of the twentieth century, and is traditionally still used today.

Opaque glass

Opacity or opaque refers to something difficult to see through or perceive. Usually frosted glass. Most times it is used on bathroom windows.

Oriel window

A window with many panels. It is most often seen in Tudor-style houses and monasteries. An oriel window projects from the wall and does not extend to the ground. Oriel windows originated as a form of porch. They are often supported by brackets or corbels. Buildings in the Gothic Revival style often have oriel windows.

Palladian Window

A three-part window opening with a tall, round-arched center window flanked by smaller rectangular windows and separated by posts or pilasters.


An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows


Sash is a frame that holds the glass assembly.


Sill is the bottom of the window frame.

Single Hung Window

One sash is movable (usually the bottom one) and the other fixed. This is the earlier form of sliding sash window, and is obviously also cheaper.


A flat or sloped window built into a roof structure for day lighting

Tilt and Turn Window

A window that can either tilt inwards at the top, or can open inwards hinged at the side.


A measure of air-to-air heat transmission (loss or gain) due to thermal conductance and the difference in indoor and outdoor temperatures. As the U-Value decreases, so does the amount of heat that is transferred through the glazing material. The lower the U-Value, the more restrictive the fenestration product is to heat transfer (Reciprocal of the R-Value).

Vinyl Windows

Vinyl windows are typically made from PVC (polyvinyl chloride), and provide very good energy savings. For the budget minded consumer, they offer a great value in price versus performance. Available in standard sizes, they’re also available in custom sizes and shapes. Vinyl is maintenance-free, since the vinyl color goes all the way through. They never need painting or touch-ups, since nicks and scratches don’t show a different color. For all these reasons, vinyl windows are popular for direct replacement window applications.

Wood Windows

Wood windows offer exceptional energy savings, and come in an almost endless variety of standard sizes and shapes. Custom sizes and shapes are also available. A number of different exterior and interior coating options (cladding) are available that make them virtually maintenance-free. Wood windows are the choice when you need to match existing windows, and will add more resale value to your home than vinyl windows.