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The Ultimate Window Glossary

Window quotations, and the window industry in general, display many terms that we do not use day-to-day. Our technical team are always available via phone or email should you have any questions.But we also understand how useful it can be able to reference these for yourself – perhaps you need an answer quickly, but it’s late in the evening.

Below, we have laid out all of the common terms that we think are not immediately clear, unless you have had some previous experience with the window industry.

To navigate faster, the blue buttons will take you to a particular section. Or, if you know the exact term you wish to find: on your keyboard, simply hold the Ctrl+f keys to bring up a search box.

We apologise if you still cannot find what you’re looking for. But please do ask us directly and let us know that we should include your query.

General Appearance

Circular, and Arched windowsNon-standard window shapes that include parts of a circle, these windows will likely be more expensive than their strait-edged counterparts. If you’re looking for arched heads, we can also to a timber, or timber-clad arched head to fill the space and leave a pleaseing aesthetic.
Raked, and Shaped WindowsThree descriptions of a non-standard window shape, raked is with a single slope, shaped for more than one slope.
RoseCircular covering behind the door handle or locking cylinder
Timber or Aluminium-clad ArchedheadsA piece of material designed and manufactured to fit above a window to fill the space where an arched window used to be, this can be helpful for saving money whilst keeping the previous style of the house.
Light or LiteA light or lite is generally used to describe a single glass pane within a slim fixed window frame
EscutcheonDecorative door handle or lock cover-plate

Performance and Insulation

Weather StrippingThe first line of defense from wind, rain, hail, snow, etc… Unlike airtight seals, this one may not meet at corner and may have strategically placed gaps so that air can flow through trickle vents.
Air Tightness Class SystemOne measure of the performance of a window is in its airtightness. The higher the class number, the more airtight the window.
Operation or Operating CyclesThe average number of times a window can be opened or closed before we would expect something to break. For some reference, a 10,000 cycle operating time means if you open a window in the morning then close it again before bed, you should expect it to last for nearly 30 years before anything breaks.
Passivhaus or Passive HouseA very high standard that a window can be built to meet. Windows that meet the approval of the Passivhaus Institute must have an overall U value of 0.8 W/m^2k or better.
Performance WindowSadly, just a common marketing term. Any product can be shown to perform to some level.
Uf ValueThe measure of how well the window frame insulates; lower values mean better insulation.
Ug’ ValueThe measure of how well the glass itself can insulate; again lower values mean better insulation.
Uw’ for ‘u’ ValueThe Uw value is the overall U value calculated for the whole window (frame and glass), and it describes how well the window insulates. The lower the number, the better the insulation. The door equivalent is “Ud”.
Water Tightness Class SystemA tested measure of how well a window can keep out driving rain. The higher the class number, the more watertight the window.
Wind Loading Class SystemA tested measure of how well a window can keep out high speed winds. The higher the class number, the more resistant the window.

Window Frames

Casement WindowA Casement window is a window attached at the side with one or more hinges, it is also commonly used to refer to any outward opening window.
CladdingThe term used to describe a difference between the main body of a window frame and the visual elements. Aluminium-clad windows are a common form of this with an external aluminium piece to reduce maintenance needs.
Composite WindowGenerally refers to any window that has a frame constructed of two or more different materials. However, it normally short-hand for aluminium-timber composite. Where the majority of frame is naturally well-insulating timber, and a thin external aluminium plate provides a maintenance-free benefit.
EspagnoletteAn espagnolette, commonly refered to as espag, is the locking mechanism for a window or door where a metal rod operates two or more locking points.
Flush finishTraditional in Scandinavia, a flush frame gives a flat external appearance – with the outer frame in-line with the opening section. The more commonly found opposite to this would be a storm-proof casement: where the opening section overlaps the outer frame.
HeartwoodTimber taken from the part of the tree closest to the tree’s “heart” or centre. Heartwood is naturally more durable, resistant to water ingress and generally has a much tighter ring structure. All in all, the best type of timber to create a window frame.
HornsA purely decorative feature, commonly found on a sliding sash window. Most sliding sash products do not have these as standard, so please ask if you’re unsure.
MouldingSounds horrible, but actually a decorative feature. Moulding has no performance benefit, but can make a modern window appear more traditional in appearance. Usually, contemporary 90 degree angles are shaped to give a more rounded and classical aesthetic.
RAL ColourYou’ve probably seen variations of “any RAL colour is available”. This is simply an extensive range of colours that almost every window manufacturer has access to. Think of the Dulux colour chart – this is the window equivalent.
SashThe sash is the part of the window frame that opens.
Sash WindowA Sash window is a window that slides up and down instead of opening inward or outward, these are commonly found on older town houses
Thermal Break and Thermally BrokenMetal is generally not a good insulating material, and yet many window frames are made from aluminium or steel. The reason why they are not completely terrible is because most are thermally broken. The thermal break is a different material (normally a plastic composite) that physically separates the inside and outside of the window frame – reducing the amount of heat loss through the frame.


Bay, Bow or Oriel WindowMore than one window configured so that there they project away from the building.
Lead TimesThe amount of time needed between the final order of the windows being sent and receiving the windows. This can vary greatly between manufacturers and even change depending on the season; so do make sure to speak to tell us early if you have a critical timing for your build.
Marine GradeNormally referring to the aluminium exterior of an alu-clad timber window, marine-grade windows have been extensively tested to prove they are suitable and long-lasting when installed near the coast.
MVHRMechanical Ventilation with Heat Recovery, this is something we might speak about if you’re looking into triple glazing. It replaces the trickle vents we would otherwise need to comply with Part F of the Building regulations. It allows for old and new air to be swapped whilst using less heat.
Overall Frame Size (FS)The dimensions of the physical window, this will be used to refer to the size of window ordered in relation to the Structural Opening.
Profile DrawingsThese are technical drawings made by a company to show how an item would look if you cut it in half. For most of our window they are available on the website under the Downloads tab.
RevealEssentially, the structural opening, but often more specifically referring to the materials around the corner from the face of a property – where you would see a cavity closer.
Structural Opening (SO)The final dimension of an opening that a window or door will be fixed into, this is important as tolerances must be taken off so that the overall dimensions can be sent to the manufacturer.


Box CillBox cills are thick cills used to give more rigidity to the base of a door and run off water
Cant BrickCant bricks are angled bricks used in-place of a window or door cill. they are most commony seen under doors, but they can look rather attractive underneath windows too.
Cill PackerCill packers are treated lengths of timber, siliconed to the bottom of a window to provide a place to attach an external cill.
DripA drip is a machined channel in the base of a window used to divert water away from the inside of the window, this helps to stop water ingress and damp spots.
Expanding FoamExpanding foam is used to create an air seal behind an expanding tape seal, the foam also holds the window rigidly in place
Expanding TapeExpanding Tape is used to create a weather seal around a window
StrapsStraps are used in new build installations to fix windows to the internal load bearing wall. Generally made from steel, a strap will be screwed into the window frame and wall bridging the airgap – allowing a fitter more flexibility in where they want to position the window.
Structural OpeningWhen speaking about the installation you may hear the words structural opening, this is used to describe the hole that will be available to put a window into.
Stubb CillA stubb cill is a window cill that is much shorter than would usually be used, these are generally used to bridge the gap between the window drip and a cant brick or stone cill
Window CillWindow cills are used to move rain away from the underside of a window where it could pool and cause damage, they should be placed behind the window drip and protrude from the house.
Security MasticSecurity mastic is used by companies to keep glass in a window, this is a construction adhesive which is very difficult to remove

Real-world Operation

Egress / Fire EgressFire Egress compliance is a part of the building regulations for all buildings in the UK, this is a window that has been sized so that a firefighter can gain entrance to the home in the case of an emergency. While we do not need to add these to renovations if the previous windows did not have the option, we would strongly advise including fire egress compliance in all bedroom windows.
Guided windowA guided window differs from a hung window as it uses rails to open, this allows friction to hold the open window in position and hides the opening mechanism at the cost of some opening area and a small price increase
Hung windowA hung window has exposed hinges on the exterior, this allows for a large opening area and cost effective construction. Many hung window also include a friction break.
EspagnoletteOur multi-point locks are operated by a single handle – this is thanks to the espagnolette that runs from the top to the bottom of the sash; linking the hook bolts to the handle.
False, Flying, or Loose MullionThis is a choice to have if you have two opening sections of a window that meet at a vertical post. The post inbetween does not have to be attached. If you ask for a false mullion, the post will move to give a large clear opening. This is often used to a fire egress compliant window, without having to change a traditional appearance.
Friction BrakeOn a side-hung window, the handle can be put into the closed position when the window is open. This prevents the window from moving in light winds. The mechanism behind this is the friction brake.
Night VentA window is put into a night vent position by opening the casement very slightly, and closing the handle down on a separate locking point. This always for a small amount of ventilation.
RestrictorChild-safe restrictors can be fitted to the window. This is allows the window to be opened a couple of inches, but no further without first releasing the catch.
Tilt-TurnTilt and Turn windows are very common in central Europe (especially Germany). They open inwards and, depending on how you turn the handle, the window will either tilt slightly towards you from the top, or turn from the side. We go into lots of detail on tilt-turn windows in the main tilt and turn article.
False SashA fixed window that looks like a normal outward opening window, these are useful for keeping everything symmetrical and lining through with other windows, also known as a dummy sash.


Bi-fold or Bifolding DoorsA Bi-fold door is a door that folds in more than one place as it opens, these are brilliant for getting the most out of a large opening. They allow most of the opening area free of any obstruction when open. Our Lacuna Bi-folds are a great example of Bi-folds done right, with high-end thermal properties and a long lasting design.
Entrance DoorsAre a door with handles and locks on both the inside and outside of the door,
Sidelights & ToplightsSidelights and Toplights are glazed frames added to doors to fill space on the sides or above a door.
Sliding doorsSliding doors are as the name suggests a sliding door, they generally slide along a rail at the base of the door
Suited KeysIf you have more than one door in your quote, it is likely that you will need to use a different key for each one. If you would prefer to use the same key for every door, ask for suited keys.
Terrace DoorTerrace doors are a door with a window handle and the option to lock the door completely, they also come with friction breaks so it is possible to keep the door open with out it blowing shut.
Tilt-Turn DoorsTilt&Turn doors are essentially an inward opening window door, except the full height can also be tilted inwards by a small amount for ventilation.
Window DoorsA Window Door is a misnomer of sorts, more realistically it is a large side-hung window with a threshold, this looses the external handle but gives much thinner frames, which is perfect for increasing the glass area.

For more information about each doors operation, style, and differences see our Bespoke Doors Page.


ArgonArgon is a gas used in glazing to reduce the heat conductivity of double or triple glazed windows. This reduction of thermal conductivity makes argon a must have for any double glazed windows.
Float GlassFloat glass is glass that has been manufactured by floating molten glass on a molten metal, this allows glass manufacturers to more easily make a glass pane that has uniform thickness.
Glazing BarsGlazing bars are internal and external glass decorations used to break up larger panes of glass visually. They are commonly used to be in-keeping with a more traditional area, whilst maintaining good airtightness.
Glazing BeadA glazing bead is a strip of material used to hold the glass into the frame of the window, this is commonly done internally as this makes removing the window from outside more difficult. External glazing beads are often used inconjunction with security mastic to achieve the higher levels of security.
KryptonKrypton, the final nobel gas seeing regular use in windows, this is by far the best gass to use in terms of thermal insulation. But the most expensive aswell.
Laminated GlassLaminated glass is two or more panes of glass with resin or vinyl film between them, this holds the glass together even when it is shattered.
Low Emissivity or Low-EGlass is “Low-E” if it has had a metal or metal-oxide coating applied to the inner pane. Thanks to the difference in wavelength between radiation from the sun and heat within a building, the coating lets heat from the sun straight through and reflects heat back into the room.
Safety GlassSafety glass is commonly used to mean toughened and laminated glass, this is used in windows and doors that are within 800mm of a floor or 300mm of a door.
Solar glassSolar glass is treated to reduce the amount of energy gained from light entering a home, this is extremely useful on large south facing windows or doors.
Sound GlassSound glass is glazing that has differing thicknesses of glass to reduce noise transmission, this is usually done in triple glazing with laminations for greatest effect.
Spacer BarA spacer bar is used to insulate the edges of a glass unit, these are generally made from plastic and are often refered to as ‘warm edge spacers’.
Toughened GlassToughened glass is glass that has been hardened through chemical or thermal treatment, this causes toughened glass to be stronger than normal glass and to break into smaller peices that are less likely to injure someone.
XenonXenon is another gas used to reduce heat conductivity of windows, though more expensive than Argon, this does allow for thinner gaps between panes within the glazing unit and better u values. Of course, this does come at an increased cost.
Externally GlazedExternal Glazing is where the glass is pressed into a window or door frame from the outside then sealed into place, this is then covered with a glazing bead. This method of glazing can help reduce frame size and allow the internal face of the frame to be unmarked. To ensure security is not compromised many manufacturers will use ‘Security Mastic’ to the glass.
Internally GlazedInternal glazing is where the glass is placed into a window or door frame from the inside and sealed into place with a glazing bead, this can give added security in comparison with an externaly glazed window.
PaneA pane is a sheet of glass or another material

Building Regulations

Background VentilationFor new build homes, there must be a system in place that allows you to control the ventilation for the whole building. The two most common ways of providing this are trickle vents and mechanical heat recovery. Both of these methods help to reduce the humidity in your home and, in turn, can reduce the level of condensation.
Fire Egress CompliantPart B of UK Building Regulations describes the requirements needed for a “habitable” window to be suitable for fire escape. A fire egress compliant window must have at least 0.33m^2 of unobstructed clear opening; 450mm clear width; 450 clear height and no more than 1100mm above the floor. Locks and restrictors (as long as there is a release-catch) may still be fitted.
Part FVentilation, regulations governing the minimum airflow a building must have, both passively and actively.
Part LConservation of fuel and Power, these regulations set a minimum standard of efficiency that buildings need to have.
Part MAcessing and use of buildings, these regulations aim to make it easier for those with disabilities to access dwellings.
PAS24PAS24 is a British security test that, if passed, will mean that a window or door is compliant with Part Q (and therefore suitable for a new build property). A window or door can be “certified to PAS24”.
Purge VentilationGenerally spoken about in building regs more than anywhere else, purge ventilation means large amounts of air can be vented from a dwelling quickly, or how much air an open window can move at once.
Building regs say you need a certain amount, and you probably want to be able to open your windows, so it’s almost always no problem – this rule is more difficult to break than adhere to.
SBDSecured By Design (SBD) is a police initiative that certifies for an additional level of security. A window or door can be “approved by SBD”.
Part Q SecurityPart Q is the approved document that describes what is required to prevent unauthorised access in new build properties via the windows and doors. Windows and doors can be “compliant with Part Q”.
Whole building ventilationMore from the building regs: whole building Ventilation is about regular air movement, from either trickle vents or a MHVR system.
This type of ventilation is important, as it stops the build-up of condensation and keeps fresh air in the building.


CtrCtr is a measure that helps adjust the dB Rw (decibel reduction) for lower frequency sounds, these can often go through materials that would otherwise absorb high frequencies.
dB or DecibelsDecibels are a measure of the pressure a sound wave produces, from 0 quieter than humans can hear, continuing up through a logarithmic scale. A small aside: for every 3 decibel increase in loudness, the amount of power in that sound is doubled. This means a 60dB sound is around 1000x lounder than a 30dB sound.
RwRw is a weighted soundproofing measurement, this is used to show how effective an insulating item will be at reducing sound levels. Rw is measured in decibels.