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Timber Window and Door Glossary

When looking to replace or repair your timber windows and doors you may come across several terms that you are unfamiliar with. This A-Z glossary guide will help answer any questions you may have.

To easily search for a specific term, from a PC click control and then F or from a Mac click command and then F, then search the term you are looking for.  If you are still unsure of any terms, feel free to contact your sales surveyor who will be more than happy to help you.


AccoyaTM – a long life modified timber sourced from sustainable forests with a Class One (very durable) durability rating.

Air-leakage (Air Infiltration) - the amount of air leaking in and out of a building through cracks in walls, windows and doors.

Air-leakage Rating - a measure of the rate of air-leakage around a window, door or skylight in the presence of a specific pressure difference. The lower a window's air-leakage rating, the better its air tightness.

Annealed Glass - standard sheet of float glass which has not been coated or heat-treated.

Arched Window – a window with a curve (arch) at the top. These may be casement, sash or fixed windows.

Architrave - Exposed moulding or framing around a window or door, on either the inside or outside, to cover the space between the window frame or jamb and the wall. In Classical architecture, the lower part of a moulded cornice. (Sometimes called casing.)

Argon - an inert, nontoxic gas used in double glazed glass units to reduce heat transfer.


Bay Window – two or more windows at an angled projection, three-unit bay windows are common in Victorian terraced houses.

Bead - a wood strip against which a casement window closes. Also, a finishing trim at the sides and top of the frame to hold the sash in a sash window.

Beehive Quandrant Catch – ironmongery option to secure sash windows (see image below).

Beehive Quandrant Catch

Bottom Rail - the bottom horizontal section of a window or door (see figures 1-3).

Box Sash – a description of a window type where the windows slide up and down.

Bow Window – windows made in a bowed shape often with bowed glass.

Brighton Catch – ironmongery option to secure sash windows (see image below).

Brighton Catch


Casement – a type of window that opens on hinges, like a door.

Casement Stay – ironmongery to keep a casement window open (see image below).

Casement Stay

Cill (also called a Sill) – the section at the bottom of the window frame directly below the moving parts of the window (see figures 2 and 3).


D-Handle – handle to lift the sashes in a sash window up and down.

Draught Proofing (also called Draught Sealing) – The process of sealing the gaps between the sashes and the window frame. Draught proofing helps reduce draughts, rattling, dust, dirt and noise.


Fanlight – a window positioned above another window or a door.

(Window) Fastener – ironmongery used to secure casement windows.

FENSA – a government authorised Competent Person Scheme ensuring complete new windows and doors meet building requirements.

Fitch Catch – lock mechanism for sliding sash windows (see below image).

Sash Window Fitch Catch

Fixed – a window that cannot be opened.

Flip Over Lock – a type of lock for casement windows (see image below).

Flip Over Lock

Frame – the framework that surrounds the moving section of the window.

French window – glazed full length opening casement windows.


Glazing Bars – the bars separating the different panes of a window. The number of glazing bars on a window will depend on the period style.

Glazing Bead - a moulding or stop around the inside of a window frame to hold the glass in place.


Hardwood – timber from slow growing trees which are generally denser and more durable than untreated softwoods. Examples of hardwood include Oak, Sapele and Mahogany.

(Sash) Horns – the protruding part of a sash window located at the bottom of the top sash and/or the top of the bottom sash. There are different styles of sash horns. Below is a picture of a standard Victorian sash horn.

Short Victorian Horn Sash Window


Insulating Glass Unit - two or more pieces of glass spaced apart and sealed to form a single unit. Also called double glazing.


Jamb – the sides of the window frame.


Lambs Tongue – a type of moulding profile (see below).

Lambs Tongue

Laminated Glass - two or more sheets of glass with an inner layer of transparent plastic. Used for safety glazing, sound reduction and security.

Lift – a handle to raise and lower the sashes in a sash window (see image below).

Sash Lift

Locking Stay Pin – a type of lock for casement windows.


Meeting Rail - the part of a sliding glass door or a sash window where two panels meet and create a weather barrier.

Mortice Plate – used alongside a window fastener on casement windows.

Mullion – a structural vertical or horizontal element between window units or sliding glass doors.

Multi-aspect – multiple windows grouped together, for example a window bay or bow window.


Obscure Glass (also called Opaque) - any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion or decorative effects.

Opener – a sash window that is opening.

Ovolo – a type of moulding profile (see below).

Ovolo moulding


Pane – a section of glass in a window or door.

Parting Bead - a narrow strip of timber that holds a sash or panel in position in a frame, keeping the top and bottom sashes apart.

Pulley – ironmongery used alongside sash cords to open and close a cord hung sash window.


Reveal – the area around where the window or door frame is fitted.

Rola Lock – a lock which prevents a sash window from opening fully (see image below).

Timber Sash Window Rola Lock


Safety Glass - a heat treated concealed glass which shatters into small pieces when hit very hard.

Sash Window – a type of window that slides open, common in period properties.

Sash Cord – the cord that holds the sash weights.

Sash Weight – the weights inside a sash window that allows the window to stay open. If there is a problem with the weights, the sash may drop when opening.

Sashes – the moving parts of a window. If required, the sashes can be replaced without replacing the frame of the window.

Secondary Glazing – a second pane of glass and frame that is fitted inside the existing window. Secondary glazing is popular in Listed Buildings as an alternative to double glazing.

Side Light – a window placed directly beside a door.

Softwood – timber from fast growing, evergreen trees, softwood is less durable than hardwood. Examples of softwood include Cedar, Fir and Pine.

Spacer bar – the bar that sits in-between the two panes of glass in a double-glazed window.

Spiral balance – an alternative to sash cords, the spiral mechanism allows a sash window to open and close by having a spring that is tensioned to counterbalance the weight of the sashes.

Staff Bead – the moulded bead that holds the sashes in place. A draught-strip can be incorporated into it.

Stile - the upright or vertical edges of a door or window.

Storm Proof Casement Window – Storm proof casement windows are designed so the edge of the window is wider than the opening so when the window is closed it forms a protective surrounding over part of the frame.


Trickle Vent – a very small opening in a window to allow for ventilation.

TricoyaTM – a sheet material made from AccoyaTM which can be used to make box window frames.


U-value – a measurement of how quickly the heat from passes through the window from the inside to the outside. Windows with lower U-values have better insulation.


Venetian window – Venetian windows have three sections, a large central window which opens and narrow fixed windows either side. They are predominantly found in larger Georgian houses.


Weatherstripping – another name for draught proofing.

Window Hardware - various devices and mechanisms for the window including catches, fasteners and locks, hinges, pivots, lifts and pulls, pulleys and sash weights, sash balances and stays.

Figure 1 (click on the image to open a larger version in a separate window)
Door Glossary

Figure 2 (click on the image to open a larger version in a separate window)
Timber Sash Window Glossary

Figure 3 (click on the image to open a larger version in a separate window)
Casement Window Glossary

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