We appreciate that a lot of the terminology we use can be quite technical. We have put together a list of what the more commonly used words mean to help you understand our products.


The moulded frame surrounding a window or door.

Bay window

Windows that project out from the wall and extend to the ground.


British Fenestration Rating Council – independent organisation who test windows & doors and issue the energy rating standards.

Bottom rail

The bottom horizontal member/piece of profile of a window frame.

Bow window

A rounded bay window that projects from the wall in the shape of an arc.

Building regulation

Building Regulations are the Government codes which set out the criteria that new building projects and refurbishment work must conform to. The parts of the Building Regulations that apply to windows and doors include Part M, Part L, Part N and Part F.  They help to make sure that buildings are safe, secure and energy efficient.


The term used to describe a conventional window, with a sash that is side or top hinged. Casement windows open outwards.

Casement window    

A window with one or more casements.


A mastic compound for filling joints and sealing cracks to prevent leakage of water and air; commonly made of silicone (external) – bituminous-acrylic (internal and can be painted over)- or rubber-based materials.


Also spelt sill – The protruding lip (outside) that directs water away from brickwork at the bottom of a window opening.

Coated glass

A window glass with an outside surface provided with a mirror reflective surface; the shading coefficient ranges from 20% to 45%

Composite door              

A robust, secure style of door that looks and feels like a traditional timber door. It’s made from an outer skin of GRP (Glass Reinforced Plastic) over an engineered timber reinforced base with a thermal insulating polyurethane foam core (CFC and HFC free)


The deposit of water vapour from the air on any cold surface whose temperature is below the dew point, such as window glass or frames that are exposed to cold outdoor air.


A secure locking system used in modern doors.

Door furniture

These are your handles and restrictors

Double glazing                 

An insulating window with air space between panes.

Dummy sash                     

or dummy vent/dummy opener/fixed opener. These are all the same name for a part of the window that is made to look like it opens (has the same frame around the glass as an opener) but its fixed and won’t open).

Easy clean hinge             

or easy clean stay. A special hinge/stay that can be fitted to a side hinged casement window to allow the opener to be opened in such a way that you can reach through the hinge side of the opener to clean to the outside of the opener.

Egress hinge                     

or egress stay. A special hinge/stay that’s fitted to ensure a side opening casement window opens as wide as possible to ensure the window can be used as a fire escape.

Emergency window       

or fire escape window. A window large enough for a fireman to climb through with breathing apparatus on (at least 0.33m2 with a min dimension either way of 450mm); each bedroom should be provided with exit windows.  This is sometimes called an egress window.

Energy rating                    

or Window Energy Rating.  A government backed scheme to measure the total energy efficiency of a complete window or door (not just the glass) includes heat loss & solar gain.


Most of the profiles used in PVC-U windows, doors, conservatories and roofline are extruded.  Extrusion is the process of pushing PVC-U pellets through a die to form a solid shape.


Devices for joining two parts together, such as screws, nails and bolts.


The placement of window openings in a building wall; one of the important elements in controlling the exterior appearance of a building.


The Fenestration Self-Assessment Scheme.  FENSA Limited is a wholly owned subsidiary of the Glass and Glazing Federation.  FENSA was set up in 1996 to ensure that window and door installers were working to the correct building regulations & to help to raise standards. It is the responsibility of the installer to provide the end user with a FENSA certificate, not the supplier of the frames.

Finish Trim                         

Interior or Exterior trim around a window unit.

Fixed Light                         

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


is a plastic laminate that is UV bonded to the PVC-U to give a wood effect in many colour options.


Lead, or similar material, provided for drainage of water and to prevent water penetration into a building.

Flag hinge                          

A door hinge which has  part fixed onto the opening sash and part fixed to the outer frame, which looks like the shape of a flag

Flying mullion                  

or floating mullion. The vertical division between 2 opening windows, which when both windows are opened is attached to one of the openers leaving an un-obstructed opening.

French window               

Two casement window sashes hinged on the sides to open out leaving no bar in the middle when both sashes are open – often used to give a fire escape.

French doors                    

Two casement door sashes hinged on the sides to open in or out – the sashes extend to the floor and serve as a door to a porch or terrace.

Friction stays                    

or hinges/stays. All casement window hinges are called stays. Friction stay relates to the fact that it holds the open window in place through friction (which can be adjusted if required).


Glass & Glazing Federation, an independent organisation who act as an ombudsman to set standards of workmanship and material quality for the glass & fenestration industry.


An elastic transparent material composed of silica(sand), soda (sodium carbonate) and lime (calcium carbonate) with small quantities of aluminium, boric or magnesia oxides.


The installation of glass in a window opening.

Glazing bead                     

A removable trim/bead that holds the glass in place.

Glazing channel

A groove cut into the sash for the mounting of the glass.


The top or upper member/piece of the windows, i.e. the top of the frame.

Head Vent                         

or trickle vent, required by building regulations for providing background ventilation in new builds and extensions


A movable joint enabling a window or door to swing open.

Interior trim                      

The inside visible moulding surrounding the interior of the window frame.

Insulated window

A window with multiple glazing that provides one or more air spaces between layers of glazing.


A vertical member at the side of the window frame.


is a plastic foil that is UV bonded to the PVC-U to give a wood effect in many colour options.

Laminated glass

Two sheets of glass with a transparent plastic sheet sandwiched between to form a pane resistant to shattering.  This is sometimes called shatter proof glass. When the glass is broken it stays in situ as it is stuck to the transparent plastic sheet.


A window; a pane of glass within a window.


Horizontal member (wood, steel or stone) over a window or door opening to support the weight of the wall above.

Locking system

A fastening device in which a bolt is secured and can be operated by a key.


or Low-E. A coating for glass surfaces which reflects radiant heat energy rather than allowing that energy to radiate through the glass surface.  The lower the emissivity of the glass the lower the heat transfer coefficient.


Vertical member between window units.

Obscure glass

Any textured glass (frosted, etched, fluted, ground etc.) used for privacy light diffusion or decorative effects.

Oriel window

A window projecting from the wall and carried on brackets, corbels or a cantilever.  Unlike a bay window, the projection of an oriel does not extend all the way to the ground.

Outer frame

That portion of the window frame which is exposed to the outdoors.


A sheet of glass for glazing a window.  After installation, the pane is referred to as a “light” or window light.

Patterned glass

One or both surfaces of glass with a decorative or privacy design (also called frosted or obscure glass).

Pencil hinge                      

or butt hinge. A door hinge which has  part fixed onto the opening sash and part fixed to the outer frame, which looks like the shape of a pencil


or PVC u (polyvinylchloride un-plasticised) A factory extruded or moulded plastic material used for window framing and exactly the same material as U-PVC/PVC (just depends if you write it in True English or European English).


Horizontal member of a window sash.


A mechanism for windows and doors that restricts the width a sash will open


Framework of stiles and rails in which the lights of a window are set (usually used to describe the opening part of a window). On casement windows you can have side hung and top hung sashes.


A compressible plastic material used to seal any opening or junction of two parts such as between the glass and a metal sash, commonly made of silicone, butyl tape or polysulfide.

Side light                            

or side panel. A fixed, often narrow glass window next to a door opening (or window).


An enduring sealing agent that resists water.

Single hung sash

A window that is similar to a double hung window except that the top sash is stationary.

Sliding sash

A window which moves horizontally in grooves or tracks. Sometimes called a Box Sash or Vertical slider


The linear object that separates and maintains the space between the glass surfaces of insulating glass.

Thermal conduction      

Heat transfer through a material by contact of one molecule to the next.  Heat flows from a high temperature area to one of lower temperature.

Thermal insulation

A material that resists heat flow.

Tilt and turn

A window system that opens inwardly in two ways.  Tilt and turn windows swing completely inwards allowing for easy cleaning.  They also tilt at the top to provide good ventilation without leaving the window fully open.

Toughened glass

or toughened safety glass. A strengthened or reinforced glass that is less subject to breakage or splintering such as glass for storm doors and some windows (toughened glass is fitted as standard in all our doors and any windows in risk areas – to comply with building regulations).


A horizontal member separating a door from a window panel above the door or separating one window above another.

Transom light

The window sash located above a door.

Trickle Vent                      

or head vent, required by building regulations for providing background ventilation in new builds and extensions


Visible moulding surrounding a window opening.

Triple Glazing

3 panes of glass separated with 2 spacers, giving a thicker unit and increased heat retention and slightly improved sound reduction.


see PVC


A value indicating the rate of heat flow through a building construction, expressed in units of Btu per square foot of surface per degree F difference between indoor and outdoor air temperature.

Vertical slider                   

A window which moves horizontally in grooves or tracks. Sometimes called a Box Sash or Vertical slider

Warm edge spacer

Used to separate the two panes of glass in a double glazed unit with a spacer made of a different material (often plastic) that transmits less heat.  This makes the window more energy efficient.


A strip of resilient material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure.

Weather tight

Sealed to prevent entry of air and precipitation into the structure.


A glazed opening in an external wall; an entire unit consisting of a frame, sash and glazing and any operable elements.

Window frame                

The fixed frame of a window which holds the sash or casement as well as hardware.

Window furniture

These are your handles, letterboxes, knocker, spy holes and door hinges

Window hardware         

Various devices and mechanisms for the window including catches, cords and chains, fasteners, locks and hinges, sometimes also covers handles and letterboxes.

Window unit

A complete window with sash and frame

Window board

The horizontal member at the bottom of the internal window frame (often called the cill or ledge)

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