Custom Windows Glossary

Versatile Wood Products spends a large portion of its professional life making custom windows work. Like a master baker blends the same ingredients to produce a culinary masterpiece, our custom window designs are grounded in extraordinary mastery of ordinary window components.

Just some of the ordinary elements that make custom windows work

Here you will find various terms with pictures and diagrams, all done by our team. We hope you’ll find this look at the history and meaning of some of the window world’s more obscure terminology informative, useful and maybe a bit entertaining.

Custom Windows Glossary 

Air Infiltration:

The amount of air that passes between a sash and a frame; Measure in terms of cubic feet of air per minute per lineal foot of crack (margin).

Air infiltration is the major cause of heat loss or gain in a home. A reduction in air leaks will provide a more comfortable environment and improve energy efficiency in the home. Some ways to prevent air from leaking through windows include using caulking or weatherstripping and replacing glazing compounds. One of the best solutions for historic homes is to have failing windows restored; windows can also be replaced. Restored windows can last many years with proper maintenance.

Argon Gas:

An inert, colorless and odorless gas used to fill the airspace between the insulating glass panes. This greatly increases the overall performance of the glass; Argon gas is less conducive to heat than air.

The glass panes are sealed to keep the gas from escaping. This ensures a consistent interior temperature as well as overall energy efficiency.  This method works for all window frames and allows for unobstructed views and reliable insulation.


A vertical member attached to the meeting edge of one door panel of a pair, bridging the opening and holding one door panel inactive, while the other panel is active; the inactive panel can be unlatched and made operable after the active panel has been opened. Also sometimes used in casement window pairs.


A mechanical device used in hung windows to counterbalance the weight of the sash during operation. These can be weight-and-pulley or spring balance.

Blind Stop:

A sash or window frame member applied to the exterior vertical edge of the side and head jamb in order to serve as a stop for window sash or screens.


A window unit that swings open from one of its vertical edges. These windows are normally operated with either a casement lock and stay bar system (the traditional hardware) or a mechanical crank system with a concealed hinge.


Exterior view of casement windows

This unusually long bank of casement windows was custom designed by Versatile Wood Products. They feature crank hardware with concealed hinges and integrated screens.


Interior view of casement windows

Here’s a look at how they operate from the interior of the space.

(excerpted from the Stroud District Council’s Design Guide for Casement Windows in Preservation Districts. Check out the full guide here.) Successor to the stone mullioned window, the traditional timber casement became the most common window type by the second half of the eighteenth century. Subdivided by glazing bars, joining together the small panes of glass, the earlier designs had the opening part of the window, the ‘casement’, made of iron with lead latticing to the glass.

By around 1840, the beginning of the Victorian period, the frames and opening casements were made entirely of timber. Windows of 6-panes per casement were the most common pattern. Designs were occasionally elaborated by the use of Gothic arches or smaller panes, especially during the mid-nineteenth century.

Here’s an example of a gothic arch from the restoration of the Sellwood Pioneer Church.

From then, though, glass technology improved. The number of panes per casement was reduced to two with one horizontal glazing bar. Traditional windows usually were no wider than about 450mm(18″) per casement.

Here are a few of the more unusual casements that Versatile Wood Products has created for clients:

Here’s a nifty arched casement.

This pair of matched casements were designed for the Timberline Lodge.

Versatile Cornerstone

Did you know that casement windows are usually the most practical choice for fulfilling egress requirements in basement renovations? They can even be designed to exactly mimic the look of a home’s double hung windows. This is to keep the exterior look consistent with the rest of the home.

Check Rail:

The bottom rail of the top sash and the top rail of the bottom sash of a double hung window that meet horizontally in the center of the unit or the two vertical members of sash in a slider window that meet in the center. This is also sometimes called the meeting rail.

Did you know: Robert Hooke, the 17th century inventor of the double hung window was also the architect who rebuilt the famous London mental hospital, Bedlam (aka the Bethlehem Hospital, Moorfields) after the Great Fire of London?

Bedlam is Europe’s oldest extant psychiatric hospital and has operated as such, continuously, for over six hundred years. Hooke kept offices in the central cupola of the hospital until his death.

Cottage Style: 

A Double Hung window with a bottom sash that is taller in height than the top sash.

The configuration commonly seen in Craftsman style houses (1900 to 1930) features the six lite upper sash paired with a unobstructed larger lower sash.

A bit about the Craftsman Style:

In A Field Guide to American Houses, Virginia and Lee McAlester explain,

The Craftsman Style was the dominant style for smaller houses built throughout the country during the period from about 1905 until the early 1920s. It originated in southern California and most landmark examples are concentrated there. Like vernacular examples of the contemporaneous Prairie style, it quickly spread throughout the country through pattern books and popular magazines. The style rapidly faded from favor after the mid-1920s; few were built after 1930. Craftsman houses were inspired primarily by the work of two California brothers – Charles Sumner Greene and Henry Mather Greene – who practiced together in Pasadena from 1893 to 1914.

From about 1903 they began to design simple Craftsman-type bungalows; by 1909 they had designed and executed several exceptional landmark examples that have been called the “ultimate bungalows.” Several influences – the English Arts and Crafts movement, an interest in oriental wooden architecture, and their early training in the manual arts – appear to have led the Greenes to design and build these intricately detailed buildings. These and similar residences were given extensive publicity in such magazines as the Western Architect, The Architect, House Beautiful, Good Housekeeping, Architectural Record, Country
Life in America, and Ladies’ Home Journal, thus familiarizing the rest of the nation with the style.

As a result, a flood of pattern books appeared, offering plans for Craftsman bungalows; some even offered completely pre-cut packages of lumber and detailing to be assembled by local labor. Through these pre-cut examples, the one-story Craftsman house quickly became the most popular and fashionable smaller house in the country.

Daylight Opening:

The visible area of glass that is seen, which is slightly smaller than the actual glass size.

Direct Glaze:

A window that is glazed directly to the frame, with no sash. Common in storefront and midcentury to modern picture window applications.

Extension Jamb:

Lumber extending from a window or door frame to accommodate different wall thicknesses.

Why do you need an extension jamb? Since the energy crisis of the 1970s brought on the demand for thicker insulation, most contemporary houses are framed with 2×6 studs. Windows, though, are generally still configured for 2×4 walls, so the jambs have to be extended to bring the window flush with the drywall.

Fine Homebuilding Magazine has an extremely detailed article on the process for properly measuring and installing jamb extensions- you can check it out here.


The horizontal and vertical members of a door or window unit which surround the sash, are used to secure the door or window unit into the rough opening and to the building, and/or to which the hinge and lock strike hardware are normally attached.

Components include:
Head Jamb: The top horizontal member

Side Jamb: the vertical side members

Sill: The bottom horizontal member

Did you know: The word “sash” is derived from the French word “chassis”  which means frame.

Legislating Frame Construction
Two pieces of English legislation affected the appearance of sash windows in London. They were imposed because exposed sash boxes were seen as a fire risk.

The 1709 Building Act stated that sash windows had to be recessed 4″ back from the outer brick-work or masonry. The 1774 Act required the sash windows box frame to be set behind the brickwork, so that only about an inch of the sash box was visible from the outside.

Insulated Glass: 

Glazing comprised of two or more panes separated by a hermetically sealed airspace. Heat transmission through this type of glass is a fraction of single pane. The space may be filled with an inert gas such as argon for additional performance.


A stationary vertical member separating multiple window units while having a continuous head and sill. Window gangs with mullions are called “mulled assemblies”.

Muntin Bar:

A slender wood profile traditionally used to divide separate smaller glass (true divided lites, or TDL). For best performance with insulated units, simulated divided lites (SDL) are used on a single piece of glass with an embedded grid pattern. For both TDL and SDL, the profiles match the profile seen on the sash detail.


The portion of a window that is separate from the frame and may be either stationary or operating, consisting of stiles (vertical members) and rails (horizontal members) to be filled with glass.

Single Glazed:

Single pane glass, as opposed to double- or triple-pane insulated glass units. Usually uses putty for the exterior glass stop.

Tempered glass:

Glass that has been heated to increase its strength. When broken, it shatters in tiny fragments, reducing the possibility of injury. Required by code in certain locations.


A frame area immediately above a door or window opening containing fixed glass, an operating sash or panel, or other filler.