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Glossary of Window & Door Terminology

1 over 1; 2 over2; 8 over 8; 4 over 1 .etc Sash windows are commonly referred to by the number of glazed panes per sash. A one over one has only one pane of glass per sash and no glazing bars. A typical Georgian design of an 8 over 8 has 8 panes of glass in each sash, separated by glazing bars. It’s not uncommon to have an uneven configurations such as 4 over 1 which are common in bay windows.



Argon Inert, nontoxic gas. Most common of the gases used to fill air spaces in double glazed units for improved insulating qualities.
Air leakage The rate of air movement around a window, door, or skylight in the presence of a specific pressure difference across it. It’s expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.
Aspect Number of views obtained from a bay window. E.g. “3 aspect bay window” is 3 windows joined at 90 degrees giving 3 views or aspects (forwards, left & right). 



May also refer to the overall view that is obtained from a window. A ‘front aspect bay window’ has views over the front garden.

Annealed glass Float glass is the most common example of an annealed glass; during manufacture it is heated to a transition point and slowly allowed to cool. When broken it forms large, sharp jagged shards that can lead to serious injury. Most building codes have restricted its use in areas considered as high risk, such as low level glazing, bathrooms and doors.
Architrave Moulding around around a window or door. They serve to cover the join between the window and the surrounding wall.
Arris Sharp edges of square cut timber are lightly sanded to remove the arris or sharp point. This enables better paint adhesion.
Astragal (Glazing bar) commonly used to describe a glazing bar in a bookcase but occasionally used in ref to window glazing bars.
Awning Window Casement window hinged at the top, that swings outward like an awning.



Bay window Grouping of windows that project from the face of the building. Square, splayed or bowed in plan. Single or double storey construction.
Bead moulding decorative moulding to the outside junction of the frame lining and brickwork.
Bead Stop Wooden strip against which a casement window closes.
Beehive Arm Fastener Sash Window Catch similar to a quadrant catch but with a turned ‘beehive’ knob.
Bonded glazing bar 2 section glazing bar to allow a double glazed unit to pass through. The outside part of the moulding is effectively a fake putty bead, bonded to the glass with a special adhesive glazing tape.
Borrowed light Any panel that allows light to be ‘shared ‘into adjacent spaces. Commonly a high level glazed panel above internal doors.
Boston hinge Small hinge fitted to staff beads that allows the bead to swing out of position, giving easy access to the lower sash.
Bottom rail Bottom horizontal member of a sash or casement.
Bottom sash Sashwindows have a pair of sashes. An upper or top sash and a lower or bottom sash.
Bow window Bow windows are curved when looked at from above, on plan. Often several standard ‘flat glazed’ windows are formed on a continuous bowed sill to create a bowed bay window. However for only the finest quality windows both the sashes and the glass are bowed to form a true bow window.
Box sash window Traditional weight balanced, vertically sliding window. Named because of the boxes built to contain the weights.
Brighton pattern fastener Sash window catch that offers better security over many traditional catches. Care must be taken on double glazed or thin sashes that the spring action does not hit the glazing..
Bull-Nose Wide semicircle profile shape. Commonly refers to the small indoor sub-cill where the architraves end.



Cambridge fastener (Meakins Fastener) Allows remote operation of the sash with the use of assistor cords and pulley’s. Ideal for difficult situations where the catch is hard to reach.
Cane H shaped piece of lead or zinc used to link pieces of glass edge to edge as in leaded lights or zinc lights & stained glass work.
Carrier (Flat or ‘T’ carrier) Brown or white uPVC strip with a channel used to fix our pile weatherstripping.
Casement Window (Mullioned window) Window that opens on hinges attached to the frame. As the sash closes by pressing against the frame compression seals may used with lower air leakage rates than sliding windows.
Casing Another name for Architrave
Clerestory Window (Clear story) Clestory windows form a narrow band and sit atop a high wall. The clerestory wall usually rises above any adjoining roofs. The word originally referred to the upper level of a church.
Corbel A corbel is an architectural bracket or block projecting from a wall.
Conservation area Area of special architectural or historic interest,considered desirable to preserve.
Condensation Mist of water vapour that forms on any cold surface whose temperature is below the dew point. Altering air-flow by installing window seals can increase condensation. Mechanical fan extraction is the preferred way to remove excess water vapour.
Counter-balance Sash weight that is attached to a cord, over a pulley; to balance the weight of the glazed sash.
Cill (Sill) Lower horizontal section of a door or window frame. Also the horizontal member beneath a window or door, usually of brick or stone construction
Cross Window 4 light casement window with a transom. Many were replaced with sash in the 18 – 19 centuries.


A projecting point formed where two curves meet. Common in Gothic architecture they add extra decoration to window tracery and sculptural carvings.



Diocletian Window (Thermal Window) is a semi-circular opening divided into three by vertical mullions.
Double glazing Two panes of glass separated by a sealed air space. This configuration offers significant improvements to the thermal efficiency of window glazing. The air space is often gas filled to further improve heat conductivity.
Double-hung (Sash window) Double hung indicates that both sashes are to operate.
Dormer window Vertical window with its own roof that projects through a sloping roof surface.
Draught excluders Weather seals attached to the bottom of doors.
Drip Groove cut into the underside of a cill to stop water running back into the building.
Drip stone (Label or hood mould) a continuous projecting moulding over a window.
Dry-glazing The use of glazing tapes, spacer blocks and wooden glazing bead instead of linseed putty.



Epoxy resin (Resin repair) Synthetic glue / filler used to make long lasting repairs on windows and doors.
Espagnolette Locking system found typically on french windows. Rods are controlled by knob mechanism having hooked ends that engage catches in the head and sill of the frame.



Façade Any side of a building, but especially an imposing or decorative one, facing a public space.
Fanlight Semicircle (or semi-elliptical) shaped sash above a door or window usually featuring radiating glazing bars, giving a fan like appearance. The term fanlight has been commonly adapted to include any small window found over a door.
Federation Australian architectural period c. 1890 – c. 1915. Federation style was, broadly speaking, the Australian version of the UK Edwardian Period.
Fenestrator Manually operated winding system to facilitate easier operation of heavy sash windows.


A word used to describe the window arrangement of a building.
Fitch fastener Sash window catch. This is the standard catch we use at Sash Window Specialist.
Fixed sashes A pair of sashes fixed shut. Often used in bay windows to maintain the aesthetics instead of using a single large glazed panel.
Fixed window Fixed panes that don’t open. When installed properly they’re airtight, but are not suitable in places where window ventilation is desired.
Flashing Weatherproofing that bridges joints formed by different materials or surfaces. Traditionally a sheet metal such as lead was used.
Float glass 90% of the worlds flat glass is now made using the Pilkington process. Glass is formed by floating the moulten glass on a bed of moulten tin. It produces a high quality glass with perfectly flat, parallel surfaces.
Frame (window) Outermost timbers of a window, jointed together and used to contain the window components as well as form a fixing with the wall.
Friction stay Side mounted hinge mechanism for casements that suspend the window open.
Frieze horizontal band that runs above doorways and windows or below the cornice. The frieze may be decorated with designs or carvings.

French window
Pair of casement windows extending to the floor and serving as portals.




Gas filled unit An inert, nontoxic gas such as Argon or Krypton may be used to fill the air space in a double glazed unit. These gases can improve a double glazed window’s energy rating by over 30%. Krypton and Xenon are better performers than Argon but much more expensive. Over time the gas does permeate the window edge seal but over a 25 year period it should not lose more than 5%.
Gothic-style Revitalist style with intricate swirling tracery and complex mouldings.
Glass A remarkable substance used by mankind for 3,500 years. The architectural glasses used for glazing windows are mostly a soda-lime-silica glass.
Glazier Tradesman who undertakes glazing work.
Glazing Glass part of a window.
Glazing bar (Muntin Bar / Astragal) Originally handmade glass could only be made in small sheets and so delicate mouldings of wood were used to join pieces of glass in a sash. The design of the glazing bars plays a very dominant role in the look of the window.
Glazing bead Putty or timber moulding that holds the glass in place.



Hammerhead Joint Used to connect an arched sash head to the sash stile.
Hardware (window) Architectural hardware including catches, stays, lifts, pulley’s, fasteners and locks but also included are sash weights & balances.
Head / header Top horizontal member of a window frame. Also used for head rail, head bead & head jamb.
Hinged windows Projection windows that utilise hinges.
Hinge Butt hinges are traditionally used on casement windows and doors. They act as a bearing between the frame and sash (or door) allowing a limited angle of rotation.
Horn (Joggle) Extended stiles of the sash that protrude beyond the rectangular frame. They add strength to the sash and can also prevent injury if the top sash drops. Less common in Scotland.
H.O.A.R.G. Hack Out And Re Glaze – replace a pane of glass.
Hook fastener Sash window catch.
Hopper window Bottom pivoting casement window that opens by tilting inwards.
Horticultural Sheet A low quality glass with a wave in one direction. Sometimes chosen as a restoration glass because of its cheap price and imperfections. It lacks the lustre of true heritage glass.
Hung A sash is said to be hung when its attached to it cord. A casement or door is hung when its fixed to the frame on hinges.



Inner Jamb Vertical member that makes up the interior face of the box frame. Joined to the pulley stile.
Intumescent seal Door seals for protection against fire & smoke.
Ironmongery Architectural Ironmongery includes catches, sash lifts, stays, locks and fasteners



Jamb Vertical side member of a door frame,window frame or lining.
Joggle Regional term for a sash horn. Joggle originates from masonry where one stone is fitted into another similar to a tenon in carpentry.



Krypton Inert, nontoxic gas. Used to fill the air space in double glazed units for improved insulating properties. Xenon and Krypton are more effective than Argon but also more expensive and so seldom used. Over time some gas permeates the window edge seal.



Lancet Arch (Gothic Arch) an arch having a head that is acutely pointed.


The glazed lantern was developed during the Middle Ages. Found on the top of buildings, lanterns are glazed structures designed to let light in to the roof and rooms below. Bigger than a single window skylight some resemble small conservatories.
Lambs tongue Moulding profile used on the inside of sashes with a deep, symmetrical profile ending in a narrow edge.
Laminated glass Manufactured by bonding multiple panes of glass together with a thin film sandwiched between the glass layers. Laminated glass has many benefits including reduced ultra violet (UV) light ingress, Grade A safety glass, improved security as well as improved noise reduction. 
Lead Heavy metal used in lead-light windows and as counter balance weight.
Leaded light  
Leaf a part of a side-hinged door system, glazed or un-glazed, surrounded by a frame. Leaves can be fixed in place (non-operable) or movable (operable).
Lift (Sash-Lift) Handle for raising the bottom sash in a double-hung sash window. Commonly a brass hook or hoop.
Light-to-solar gain (LSG) The ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn’t always provided.
Lintel Horizontal load bearing beam over the top of a window, also known as a transom.
Listed building Building that has been designated as being of special architectural or historic interest. Consent must be obtained before considering any type of window works.
Lattice light Window pane made-up of small pieces of glass held together in a lattice formation.
Light (Panel / lite) The glazed section of a window within a frame of fixed division i.e. mullions and transoms. Lights can be divided into panes. The terms “single-light”, “double-light” etc. refer to the number of these glass panes in a window.
Louvre Window Jalousie window) is a window which consists of parallel glass louvers set in a frame. commonly the louvers are connected together by a track, so that they may be tilted open and shut in unison. Some lovre windows are fixed in place however providing continous ventilation..
Low-E Glass (low thermal emissivity) -A coating is applied to the glass during production that results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside. Infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.


Make-weight Add on weight, usually of lead, with a central hole for the cord to pass through. Used to make up the weight difference after re glazing work.
Marginal lights or glazing bar Glazing bars set closer to the edge. Commonly the marginal lights have ornate glasses creating a decorative border within the sash.
Meakins fastener (Cambridge fastener) Allows remote operation of the sash with the use of assistor cords and pulley’s. Used for situations where the catch is difficult to reach.
Meeting rail (Mid rail / check rail) Horizontal rail of each sash that meets a rail of the other when the window is closed. The catch is located here.
Microporous Paint that allows moisture vapour out of the timber but prevents rain drops getting in.
Mitre 45 degree cut.
Mortice (Mortise) The female socket in a mortice and tenon joint. A strong joint used to construct traditional sash frames.
Monkey-tail Antique spiraled design on the ends of black iron casement fasteners and stays.
Moulding Shapes or contour profiles cut into the timber for decorative purposes. (see Ovolo & Lambs tongue)
Mouse (sash) A slim weight attached to a thin string that is passed over the pulley and used as a weighted drawstring for the sash cord.
Mullion Vertical member dividing the lights of a window. Named mullions when load-bearing, muntins when not. The name was adapted from the thin stone supports found in medieval and Tudor buildings that divide up the glass panels and help support the structure above.
Muntin Vertical member dividing the lights of a window. Named mullions when load-bearing, muntins when not.



Nosing Protruding edge, usually with a bull nose profile.



Obscure glass Any glass that has had the clarity obscured usually with either a textured pattern or etched surface. Used for privacy, light diffusion, or decoration.
Oculus Window (‘oeuil de boeuf’ or bulls eye) Circular window popular in European Baroque architecture.


(Mashrabiya) Oriels project out from a wall, supported below on a bracket or corbel and are commonly glazed on three sides.Common in medieval and Tudor architecture, they became fashionable again in the Victorian period.
Ovolo Moulding profile used on the inside of sashes. A convex cross section approximating a quarter of a circle or ellipse.



Paint Lead, Oil or acrylic based. It serves as a skin to protect and enhance the windows.


Palladian Window (Venetian) Tripartite window usually with centre light arched and larger than flanking lights. The term “Palladian” comes from Andrea Palladio, a Renaissance architect whose work inspired some of the greatest buildings in Europe and the United States
Pane Single sheet of glass.
Panel (light) The glazed section of a window within a frame of fixed division i.e. mullions and transoms. Lights can be divided into panes. The terms “single-light”, “double-light” etc. refer to the number of these glass panes in a window.
Parting bead (parting stop, parting strip).A long narrow strip running vertically between the upper and lower sashes, enabling them to slide past each other
Pile weather-stripping Brush type weatherstrip with a central fin that acts as a moisture barrier. Low friction & ideal for sliding sash windows.
Pediment A low-pitched triangular gable on buildings.
Pivot window Casement that is hinged with one pivot hinge each side allowing the sash to be spun 180 degrees.
Pocket Access panel to the weight box. Usually on the pulley stile.
Polished chrome (Chrome plated brass) is a shiny finish used on architectural ironmongery
Polished brass Shiny lacquer finished applied to polished brass architectural ironmongery
Projected windows As the sash opens it is projected outwards beyond the plane of the window. Casement, awning windows and hopper are all projected windows.
Plate glass Flat glass sheet (plate of glass) used for windows & doors. Most window glass is soda-lime glass produced by the float glass process. Polished plate was a high quality, expensive hand made glass.
Primer First coat of paint applied to wood. It ensures a good bond between the timber and painted top coats. Some primers may contain metals such as aluminium that make the paint much more durable.
Pulley Used in the balance mechanism of box sash windows to ensure smooth operation. Maybe a pressed, axel or ball bearing type. Available with square or round ends
Pulley stile The member of the box that contains the pulleys and a groove for the parting bead. Its most common for the pocket to be in this member.
Putty Linseed oil putty is a glazing compound used to seal the join between glass and wooden frame. A putty knife is used to cut and smooth the finish. Butyl putty is a non setting compound used to bed in glazing.



Q-Lon Foam based weatherstip made by Schlegel
Quadrant arm fastener Sash window catch with an arm that swings through a quadrant to close.


A pattern used in masonry and wood that is similar to the shape of a four-leafed clover and often featured as part of window tracery. A quatrefoil window is a round window that is composed of a quatrefoil design with four equal lobes.



Rail Horizontal sash component. Top rail, bottom rail & meeting rail.
Rebate [aka. Rabbet] Recess or a square cut longitudinal groove. Rebates are moulded into the sash frame to retain the glass.
Reveal Vertical side of an opening in a wall, especially between the window or door frame and the front of the wall. Splayed or square, maybe plaster or timber finished.
Rola locks Security stops for Sash Windows.
Roof light Window that follows the roof line.


Saddle bar Horizontal metal bar used to strengthen leaded lights.
Safety glass Toughened or reinforced glass that is less subject to breakage or forming dangerous shards.
Sash Frame to hold the glass that may be fixed or operable. Sometimes called a casement when hinged.
Sash box Window frame for a box sash window, into which the sashes slide. Characterised by the weight boxes located on either side.
Sash chain Less common option for hanging the sashes is to use chain instead of cord for greater strength.
Sash eye (sash pull) Type of sash lift with a ring shape.
Sash handle D shaped handle fitted externally to the bottom rail of the top sash.
Sash lift Handle for raising the bottom sash in a double-hung sash window. Commonly a brass hook or hoop.
Sash screw Traditional security device that bolts the meeting rails together.
Satin chrome (Chrome plated brass) is a matt finish used on architectural ironmongery
Satin glass Opaque finish. Originally this effect was achieved by sand blasting the glass surface
Satin nickel Matt chrome finish used for architectural ironmongery
Scarf Joint (Half-lap) When old timber is cut away and new timber attached a scarf join is used.
Secondary glazing Additional glazed frame applied to the inside of an existing frame, often used on protected or listed buildings to achieve higher levels of thermal and sound insulation without compromising the look of the building.
Segmental arch:  Arch made of less than half of a circle, the curve ending sharply. An elliptical arch.
Sheet glass Transparent, flat glass found in older windows, now largely replaced by float glass
Shading coefficient (SC) The fraction of solar radiation admitted through a window, door, or skylight — either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Your home’s climate, orientation, and external shading will determine the optimal SHGC for a particular window, door, or skylight. 
Shutters (aka. Leaves)Originally ‘shutting windows’, the key difference between a shutter and a window is the use of glazed lights. Wooden window shutters are typical fitted to the interior and offer security, insulation and privacy.
Single glazing Single thickness of glass in a window or door.
Sidelights Fixed doors or sashes adjacent to operable windows or doors allowing in extra light.
Single-hung A sash window with one of the sashes fixed shut & 1 functional..
Sill (Cill) Lower horizontal section of a door or window frame. Also the horizontal member beneath a window or door, usually of brick or stone construction
Sliding window Sliding window types include sliders, single-hung windows, double-hung windows & Yorkshire sash. As the sash opens it remains in the same flat plane.
Slip-head window (Box-head / Jib-head) If a bottom sash is larger (typically 2/3) sometimes a pocket for the lower sash is formed into the wall above. When the lower sash is in the fully up position, it equals the top sash.
Soldier course Row of bricks placed on end above a window or door opening. Either laid horizontal or as an arched sprung soldier course.
Spacer bar Used to separate the panes of glass in a double glazed unit.
Spring / spiral balance Spring balances offered a cheaper way to manufacture vertical sliding sash windows. There are several variations of these balances but they all utilise a spring to offset the weight of the sash.
Stile The vertical members of a sash that are jointed to the rails.
Staircase Window Very elongated sash used in stairwells. the top is commonly fixed shut.
Stop The moulding on the inside of a window frame against which the window sash closes; in the case of a double-hung window, the sash slides against the stop. Also called bead, side stop, window stop, and parting stop
Storm proof Design of casement window developed in the 60’s that incorporates a flange on the exterior of a casement that overlap the jamb, head, mullion or transom sections of the frame.
Staff bead (sash stops, sash beads, baton rod) Narrow bead that fits to the inside edge of a sash window. It forms the channel with the parting bead for the bottom sash to travel in.




Tape balance Similar to an everyday retractable tape measure in design, the end of the tape connects to the bottom of the sash. As the window opens the tape retracts into the balance.
Tinted glass Glass coloured by incorporating additives during manufacture.
Tempered glass Glass that is strengthened by reheating it to just below the melting point and then suddenly cooling it. When shattered, it breaks into small pieces. Approximately five times stronger than standard annealed glass; is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations. It cannot be re-cut after tempering.
Tenon Male part of a mortise & tenon joint. A strong joint used to construct traditional sash frames.
Top sash sashwindows have a pair of sashes. An upper or top sash and a lower or bottom sash.



Fine decorative carving in either wood or stone found in the design of windows, vaults, screens and panels.
Transom A horizontal member of timber or stone. Commonly used to describe a horizontal beam morticed into the frame separating the lights of a window or door frame.
Transom light (Transom window or borrowed light). Small window located above a transom.
Travel Refers to the distance the sash and weights can travel in a vertical direction. In some instances the requirement for heavier weights might cause a reduction in travel.
Treble sash A window having three vertically sliding sashes, one above the other, each of which closes a different part of the window; once used in large houses having very high ceilings
Trickle vent Surface mounted ventilation, allows controlled air flow..
Tripartite Window composed of three panels / lights within the frame.
Triple glazing Three panes of glass with two air spaces.



U-factor (U-value) U-factor is the rate at which a window, door, or skylight conducts non-solar heat flow. It’s usually expressed in units of Btu/hr-ft2-oF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.
uPVC (Unplasticized polyvinyl chloride, rigid PVC, Vinyl) A durable rigid plastic used for making windows and components. At Sash Window Specialist we use a uPVC carrier to hold our pile weatherstripping.





Vernacular architecture is the term used to indicate that the architecture is local to the region in which it is found and generated by the people of that region. The design is often produced by the work of craftsmen and builders rather than architects, and buildings are made of locally produced materials usually using traditional building methods.
Venetian Large window composed of three panels / lights horizontally divided. The central light is usually larger and sometimes features an arched head. Other variations of the venetian have a triangular or flat top.
Victorian sash lift A rail type sash lift
Visible transmittance (VT) is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window, door, or skylight. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home’s day lighting requirements and/or whether you need to reduce interior glare in a space.



Wagtail (Waggle board, Tongue) Sliver of wood suspended in the weight box. It prevents collision of the weights.
Waxed cord Traditional cotton cord for use in double hung windows. Most waxed cord now has a polyester core for improved durability.
Weatherseals (Weather strips, draught proofing) A variety of products used to reduce airflow through an opening window. Their effectiveness is measured with the air tightness test. Often described by their appearance in cross section ‘P’ , ‘Z’ and ‘E’ flipper or bubble….
Weekes stop Security device for sliding sash windows.
Weight (sash) Most traditional box sash windows used cast iron weights, secured with a knot tied in the cord. With the desire to increase glazing thickness replacement weights are cast from lead, a much heavier metal.
Window Opening in the wall of a building for the admission of air, light or both. Commonly fitted with a frame in which are set movable sashes containing panes of glass.
Wyatt Window Tripartite window resembling a serilana, but with the arch omitted. Named after its inventor, James Wyatt.



Xenon Inert, nontoxic gas. Used to fill the air space in double glazed units for improved insulating properties. Over time some gas permeates the window edge seal. Xenon and Krypton are more effective than Argon but also more expensive and so seldom used.



Yorkshire sash Horizontal sliding sash window dating from early 18 century. Commonly with exterior timber shutters.



‘Z’ strip Type of weathersealing. Z shaped in profile.


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A Glossary of Terms Used in Grecian, Roman, Italian and Gothic Architecture: Published by John Henry Parker, 1845, Oxford


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