Window Terminology Explained For Homeowners

Planning new windows should feel like an improvement to your home, not like learning a new language. Yet as soon as words such as sash, mullion, astragal bar or U value appear on a drawing or a quote, many homeowners find themselves nodding along and hoping the result will be right.

It does not have to work that way. Most window terminology is straightforward once someone explains it clearly, and understanding the key terms means you can read a specification, compare quotes accurately and describe what you want with confidence, without needing a technical background.

This guide covers the main parts of the frame, the sash and the glass, the most common window styles and how they open, the decorative details that matter most on period properties, and the performance terms you will encounter on any modern window specification.

Frame And Structure Terms

The frame is the foundation of every window. Once you understand its main parts, a lot of other terminology starts to fall into place.

Frame

The frame is the fixed outer structure of the window, secured to the wall of the building. It supports the moving parts and the glass, and forms the outline of the opening. In a timber window, the frame is made up of four sections: the head at the top, the cill at the bottom, and the two jambs running vertically on each side.

Head

The head is the horizontal section at the very top of the frame. It takes the load from above and, together with the cill and jambs, forms the outer perimeter of the window unit.

Cill Or Sill

The cill, sometimes spelled sill, is the horizontal section at the very bottom of the frame. It is angled slightly downward to channel rainwater away from the wall and prevent damp getting into the structure. On a timber window, the cill takes more weather exposure than any other part of the frame and is worth paying close attention to in any specification.

Jamb

The jambs are the two vertical sections running up either side of the frame, connecting the head at the top to the cill at the bottom. They carry fixings, seals and the hardware needed for the window to open and close, including hinges, locking mechanisms and stays. If you ever need to describe a draught or a problem to your joiner, jamb is the word for the vertical sides of the frame.

Mullion

A mullion is a vertical structural member that divides a wider opening into two or more separate sections. A window with three sashes side by side in one wide opening will typically have mullions between them. The mullion is structural rather than decorative and contributes to the rigidity of the whole frame.

Transom

A transom is a horizontal member that divides a tall opening into upper and lower sections. It is often used when a smaller fixed light sits above a larger opening casement within the same frame, or to add a fanlight above a door. Where a mullion divides a window left to right, a transom divides it top to bottom.

Sash And Moving Parts

The sash is the part of the window that holds the glass and, where the window opens, the part that moves. Understanding how the sash works makes it much easier to identify what style of window you have or what you are being quoted for.

Sash

The sash is the inner framework that holds the glass. On a casement window it is hinged to the frame and swings open. On a sliding sash window it moves up and down within the frame. On a fixed window the sash holds the glass in place but does not move. Most windows have one or two sashes, though larger windows can have more.

Rail

Rails are the horizontal members of the sash. There is a top rail at the upper edge and a bottom rail at the lower edge. On a sash window with two sliding panels, the point where the upper and lower sashes meet in the middle is called the meeting rail or check rail.

Stile

The stile is the vertical side piece of the sash. It runs from the top rail to the bottom rail on either side, forming the outer edge of the sash. The stile is where hinges are typically fixed on a casement window, and on a sash window it slides within channels in the frame. If you say there is damage to the left hand stile, a joiner will know exactly where to look.

Glazing Rebate

The glazing rebate is the shaped groove within the sash where the glass sits. It is sealed to keep water and air out of the joint between the glass and the timber. On a well made window, this detail is one of the things that determines how long the window performs well before it needs maintenance.

Glass And Glazing Terms

Glass does a great deal more than simply let in light. The number of panes, the gap between them, the gas inside that gap and any coatings on the surface all affect how warm the room feels, how much noise enters and how much energy the building loses through the window.

Acoustic Glazing

Acoustic glazing is designed to reduce noise transmission from outside. It typically uses thicker glass, different thicknesses for the inner and outer panes, or a laminated interlayer that absorbs sound. It is worth considering for homes near busy roads, railway lines or flight paths, where standard double glazing may not reduce noise to a comfortable level.

Argon Filled Unit

An argon filled unit is a double or triple glazed unit where the cavity between the panes is filled with argon gas rather than air. Argon is denser than air and conducts heat less readily, which improves the thermal performance of the unit. Most modern double glazed units are argon filled as standard, though it is worth confirming this when comparing specifications.

Double Glazing

A double glazed unit consists of two panes of glass separated by a spacer bar, with the cavity sealed and typically filled with argon gas. The sealed cavity reduces the rate at which heat passes through the glass, improves thermal performance and reduces condensation on the inner pane. Double glazing is now standard in virtually all new window installations in the UK.

Glazing Bar

A glazing bar is a slender member that divides the glass into smaller panes within the sash. In traditional timber windows, glazing bars are structural and hold individual panes of glass in place. In modern windows, bars are sometimes applied to the surface of a larger sealed unit to give the same appearance. The distinction matters on listed buildings and in conservation areas, where planning authorities often require true structural glazing bars rather than surface applied alternatives.

Laminated Glass

Laminated glass is made by bonding two or more panes of glass together with a transparent interlayer. If the glass breaks, the interlayer holds the fragments in place rather than allowing them to fall. It is commonly used in ground floor windows, roof lights and any application where safety or security is a priority.

Low Emissivity Glass

Low emissivity glass has a microscopic metallic coating applied to one surface that reflects heat back into the room rather than allowing it to escape through the glass. In winter this helps retain warmth inside. In summer it reduces unwanted solar heat gain. The coating is invisible to the eye and does not affect the appearance or clarity of the glass.

Single Glazing

Single glazing has one pane of glass with no sealed cavity. It offers minimal insulation and is generally found only in older, untouched windows or in certain heritage applications where the original character of the window must be preserved.

Solar Control Glazing

Solar control glazing limits the amount of heat from the sun that passes through the glass while still admitting natural light. It is particularly useful in south facing rooms, conservatories and glazed extensions where summer overheating can be a problem. It differs from low emissivity glass in that its primary aim is to reduce solar gain rather than retain internal heat.

Triple Glazing

Triple glazing uses three panes of glass with two sealed cavities between them. It offers better thermal and acoustic performance than double glazing and is worth considering in very exposed locations, properties with high energy efficiency requirements or rooms where external noise is significant. The additional weight of a triple glazed unit needs to be accounted for in the design of the frame and the specification of the hinges.

Window Styles And How They Open

Window names can be confusing because they often describe how a window opens as much as how it looks. These are the styles you are most likely to encounter on a quote or a set of drawings.

Box Sash Window

A box sash window has sashes that slide vertically within a deep boxed frame. Inside the hollow frame on each side, concealed counterweights hang on cords that run over pulleys, balancing the weight of the sash so that it stays open at any position without needing a catch or stay. The box sash is the traditional form associated with Georgian, Victorian and Edwardian architecture, and the mechanism has worked reliably for centuries.

Casement Window

A casement window is hinged and opens outward, in the same way a door does. It is the most common window type in British homes and suits a wide range of architectural styles. Within casement windows there are several important variations worth knowing.

A side hung casement is hinged at the side and opens like a narrow door. This is the most common casement type.

A top hung casement is hinged at the top and opens from the bottom. Because the sash projects outward at the base rather than the side, rainwater is shed away from the opening, making this style well suited to exposed locations and rooms where you want ventilation in wet weather. Top hung casements are often used in garden rooms and orangeries.

A bottom hung casement is hinged at the bottom and tilts inward from the top. It is less common and more often found in utility spaces where an outward opening window is not practical.

A fixed casement does not open at all. It is used purely to admit light and is often combined with opening casements in the same frame.

Flush Casement Window

A flush casement is one where the sash sits level with the face of the frame when closed, giving a flat, refined appearance. This style is closely associated with cottages, medieval buildings and period properties, as well as contemporary designs where a clean, minimal look is wanted. Flush casements require tighter tolerances than lipped casements, as the weather seal relies on the fit between the sash and frame rather than an overlapping lip.

French Casement Window

A French casement, sometimes called a double casement, has two side hung sashes that meet in the centre of the frame without a central mullion. Both sashes open outward, creating a wide, unobstructed opening with no vertical bar in the middle. The result is elegant and particularly well suited to garden facing rooms and period properties where light and proportion matter.

Lipped Casement Window

A lipped casement, also known as a stormproof casement, has a sash that slightly overlaps the outer face of the frame when closed. This overlap creates a weatherproof seal that keeps out wind and rain, which is why the style became the standard window design across most of England through the twentieth century. Lipped casements remain the most widely used window type in the country.

Sash Window

A sash window is one where the glass is held in one or more moveable panels that slide within the frame rather than opening on hinges. Most sash windows in the UK slide vertically. A single hung sash window has one moving sash and one fixed sash. A double hung sash window has two sashes that both move independently, giving more control over ventilation.

The arrangement of panes within a sash often reflects the period of the building. Georgian sash windows tend to have multiple small panes arranged in a grid, typically six over six. Victorian sash windows often place a larger pane in the lower sash with smaller panes above. Edwardian sash windows generally use fewer glazing bars and larger panes to bring in more light.

Spring Sash Window

A spring sash window uses modern spring balances concealed within the frame rather than the traditional cord, pulley and weight system of a box sash. The result looks similar from the outside and the spring mechanism is lighter to install and easier to service. It is a practical option where the full depth of a box frame is not possible or not wanted.

Tilt And Turn Window

A tilt and turn window opens in two ways from a single handle. Turning the handle one way allows the top of the sash to tilt inward for ventilation. Turning it further allows the sash to swing fully inward from the side for cleaning or emergency access. These windows are more commonly found in uPVC and aluminium systems than in timber, though they are used in some timber applications where inward opening is a practical requirement.

Decorative And Heritage Details

The finer details of a window have more effect on the character of a property than any single headline feature. On a period home in particular, getting these details right is what separates a replacement that looks as though it belongs from one that clearly does not.

Astragal Bar

An astragal bar is a decorative moulding applied to the outer surface of a sealed glazed unit to give the appearance of traditional divided lights. Unlike a structural glazing bar, it does not separate individual panes. Astragal bars are a cost effective way to achieve a period look on a modern double glazed unit. For listed buildings and conservation area work, it is worth checking with the relevant planning authority before specifying, as some will insist on individually glazed panes in a true structural bar rather than a surface applied alternative.

Georgian Bar

Georgian bars are glazing bars arranged in a regular grid pattern, replicating the appearance of the multi pane windows that were standard before large sheets of glass became affordable. The term is sometimes used interchangeably with astragal bar, though strictly speaking Georgian bars describe the layout and pattern rather than whether the bars are structural or applied. Matching the original bar width and profile on a period property makes a significant difference to how the finished window sits alongside its neighbours.

Horn

The horn is the small projection of the stile that extends below the bottom rail of the upper sash on a Victorian sash window. It was introduced in the nineteenth century when heavier panes of glass put greater strain on the corner joint of the sash, and the horn was added to strengthen it. The presence of a horn is one of the clearest ways to distinguish a Victorian sash from an earlier Georgian one.

Leaded Glass And Leaded Lights

Leaded glass uses thin strips of lead, known as came, to hold individual pieces of glass together in a decorative pattern. When this technique is used across a whole window or panel, the result is called a leaded light. The most common patterns are diamond quarries, associated with Tudor and Arts and Crafts architecture, and rectangular lights more typical of Georgian and Victorian work. At Turner & Co, leaded glazing is available across our casement range for homeowners who want to retain or recreate the authentic character of a period property.

Ovolo Moulding

The ovolo is the convex curved moulding profile worked into the edges of the frame and sash members on a traditional timber window. It is the detail that most clearly separates a well made period casement or sash from a modern uPVC equivalent. The profile is cut directly into the timber rather than applied separately, and the quality of the ovolo is a reliable indicator of the overall standard of the joinery.

Slim Sightlines

Sightlines refer to the visible width of the frame and sash members when the window is closed. Slimmer sightlines mean less frame material is visible and more of the opening is glass, giving a lighter, more open appearance. Slim sightlines are associated with contemporary design and with quality traditional timber joinery, where the ability to work to fine sections is a mark of craftsmanship.

Performance And Technical Terms

Performance terms can look daunting on a specification, but they relate to straightforward questions: will the room feel warm? Will there be draughts? And what effect will the windows have on energy bills? Understanding even the basics helps you make better decisions about energy efficiency and long term running costs.

Air Leakage And L Value

Air leakage measures how much air passes around the edges of a window when it is closed. The L value is the technical measure of this. A lower L value means a more airtight window, fewer unwanted draughts and better control over ventilation. L values are particularly relevant in new builds and major renovations where airtightness targets form part of the building regulations submission.

G Value

The G value, sometimes called the solar factor, measures how much solar heat passes through the glass. A higher G value means more heat from the sun enters the room, which can be welcome on a cool north facing wall but less so in a room that already overheats in summer. On south facing glazing or in a conservatory, a lower G value is usually preferable.

Thermal Break

A thermal break is a layer of low conductivity material placed within a frame to stop heat passing straight through from the warm interior to the cold exterior. It is most commonly associated with aluminium frames, where the metal itself conducts heat readily, but the principle applies wherever the frame material would otherwise create a cold bridge that undermines the thermal performance of the window. Timber, by contrast, provides a natural degree of thermal resistance as part of the material itself, which is one reason well made timber windows typically outperform aluminium in thermal comparisons.

U Value

The U value measures the rate at which heat passes through a window. The lower the U value, the better the window insulates. UK Building Regulations set maximum U values for windows in new builds and certain replacement situations. A well specified timber window with a modern double glazed argon filled unit can achieve U values comparable to standard uPVC products, while retaining the character and longevity of a natural material.

WER

WER stands for Window Energy Rating. It is a scale running from G at the lowest to A++ at the highest that summarises a window’s overall energy performance by combining the U value, G value and L value into a single score. WER ratings appear on window product literature and are used in building regulations compliance documentation.

Putting The Terms Into Practice

Once you know what the main terms mean, reading a window specification becomes considerably more straightforward. A quote that lists a flush casement with an ovolo profile, low emissivity double glazed argon filled units, a U value of 1.4 and astragal bars is describing a precise and well defined window. Understanding each of those terms means you can compare it accurately with alternatives, ask the right questions and feel confident that what gets built is what you actually wanted.

The same applies when speaking with a planning officer about a listed building or conservation area application. Knowing the difference between an applied astragal bar and a true structural glazing bar, or between a box sash and a spring sash, puts you in a much stronger position in that conversation.

You do not need to remember every term in this guide. What matters is that you recognise the key phrases and feel comfortable asking whoever is across the table to explain anything that is not clear. A good joiner should be happy to talk through options in plain language, as many times as needed.

Bespoke Timber Windows From Turner & Co

At Turner & Co Bespoke Joinery Ltd, all of our windows are made from premium sustainably sourced timber in our Ipswich workshop. As a family run business with deep roots in traditional timber joinery, we design and build to the specific requirements of each property, whether that is a flush casement for a period cottage, a box sash for a Georgian townhouse or a contemporary window for a new build where the client wants the warmth and quality of real timber. We manage everything from initial design and technical drawings through to spray finishing and installation.

We work with homeowners across Suffolk, Essex, Norfolk, Cambridgeshire and London. If you are planning new windows and would like to discuss your project, call us on 07952 907 946 or email harry@turnerandco.co.uk. You are also welcome to visit our Ipswich workshop to see examples of our work in person.

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