Reference
Window restoration
glossary.
An alphabetical reference for the vocabulary of sash-window work — parts of the frame, glazing types, conservation terms, and the traditional joinery details that come up on site.
A
5 entries
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Accoya wood
Accoya® wood stands as a top-quality timber product which comes from sustainable sources and undergoes acetylation to improve its natural characteristics through a non-toxic process. The treatment process provides Accoya® wood with exceptional durability which lasts more than 50 years for outdoor applications and superior dimensional stability that prevents swelling and warping and strong resistance to rot and insect damage without the need for toxic chemicals. The material selection for heritage joinery specialists Six Over Six Windows depends on its proven ability to maintain its appearance through paint preservation and its ability to resist weathering for extended periods thus making it suitable for high-performance window and door applications.
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Annealed glass
Annealed glass is standard, untreated float glass that has been cooled slowly in a controlled process (annealing) to relieve internal stresses. The material exists in translucent form while providing an affordable solution for applications which do not require safety features. The material breaks into dangerous large pieces which makes it inappropriate for use in safety-sensitive areas such as doors and windows near the ground. Building codes for these areas require the use of either tempered glass or laminated glass products. The restoration process of heritage windows uses annealed glass because it maintains the authentic look of single-glazed sash windows while providing both historical accuracy and minimal safety risk.
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Apron
The horizontal timber board fixed directly beneath a window sill on the interior — a finishing piece that covers the joint between sill and wall.
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Architrave
The decorative moulding that frames a window or door opening, covering the joint between the frame and the surrounding wall.
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Awning windows
An outward swinging window unit opening from the bottom and hinged at the top.
B
4 entries
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Bay window
A building element which projects outward from the exterior wall, typically formed of three or more sashes set at angles.
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Beading
Also called glazing bead — the thin strip of timber or moulding that holds glass panes within a sash or frame.
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Bottom rail
The lowest horizontal member of a window sash, forming the base that supports the glass and seats against the cill.
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Bow sash window
The bow sash (or bow-faced sash) is a unique and particularly attractive form of sash window. Existing in a few different formats depending on the window’s age, types include both the true bow-faced sash (incorporating bent glass which provides the curved effect) and also units where the panes of glass are flat but the curve of the frame achieves the bow effect. Providing excellent light penetration and a lovely focal point in rooms, they are often found with matching curved seating areas built around their inside face.
C
3 entries
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Casement window
A window hinged on one side so it swings open like a door — the dominant pre-Georgian English window type, and still common in cottages.
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Conservation area
A planning designation applied to streets or districts of special architectural or historic interest — replacement work requires consent.
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Conservation glazing
Conservation Glazing refers to high-performance window upgrades-such as slimline double glazing or vacuum insulated glazing (VIG)-specifically designed for use in listed buildings and conservation areas. The solutions replicate traditional single glazing by matching its appearance and thickness and sightlines while delivering enhanced performance in thermal efficiency and noise reduction and condensation resistance. The systems enable original timber sash or casement windows to function properly while preserving their structure in accordance with Historic England and local planning authority guidelines which support maintaining original windows in heritage buildings.
D
3 entries
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Dormer window
A window set into a sloping roof — a recognisable feature of historic European architecture, used to bring light into attic spaces.
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Double glazing
A window construction using two glass panes separated by a sealed air or gas gap — the standard performance upgrade for period frames.
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Draught-proofing
Sealing the moving gaps of a sash window to stop cold air ingress — usually a routed brush seal at the meeting rail and stiles.
F
4 entries
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Fitch fastener
The Fitch fastener exists as a discreet window lock which applies directly to surfaces and serves heritage sash and casement windows. The 19th century brought about this invention which uses a basic brass or steel lever system to connect with a keep on the opposite frame for secure locking that allows easy release without needing to create holes in glazing bars or historic timber. The device features a low profile together with its classic design which makes it a popular selection for Conservation Areas and Listed Buildings because these locations demand authentic preservation with minimum disturbance to the structure.
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Fixed sashes
Fixed sashes are non-opening window units-typically timber-framed panels with glazing-that are permanently sealed within a window frame or bay. The windows create unbroken views through their design which enhances thermal insulation because of their airtight construction and structural stability. The heritage preservation of fixed sashes emerges through their placement at bay window centers and door tops and decorative transoms where they mimic original designs by using authentic glazing bars and putty glazing while supporting modern insulated glass for Part L compliance.
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Fixed window
A stationary glazed panel that admits light and view but does not open — used where ventilation is provided elsewhere.
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Float glass
The standard, high-clarity flat glass used in most modern windows — made by floating molten glass on a bed of liquid tin.
G
5 entries
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Gas-filled unit
A gas-filled unit is a type of double or triple glazing where the space between panes is filled with an inert gas-typically argon, and sometimes krypton-to reduce heat transfer and improve thermal insulation. These gases are denser than air, slowing conduction and lowering the window’s U-value by up to 30%. In heritage windows, slimline gas-filled units (14–16mm thick) offer a practical balance of energy efficiency and authenticity, though they require high-quality edge seals to prevent long-term gas leakage.
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Georgian sash
Georgian sash windows are characterised by their elegant, symmetrical design and multi-pane glazing-most iconically in a six-over-six configuration…
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Glazing bar
A glazing bar serves as a horizontal or vertical bar which functions structurally or decoratively to divide window sashes into multiple glass panels.
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Glazing rail
A glazing rail is the horizontal timber member within a sash window that holds the glass panes in place, forming the top or bottom edge of each glazed section. In traditional sash construction, glazing rails work with stiles and glazing bars to create the characteristic pane layout (e.g., six-over-six). They are typically rebated to receive the glass, which is bedded into the rebate and finished with a weathering fillet dressed to a bevel — the detail that gives a period window its slim sightline.
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Grilles
Also known as glazing bars or muntins — the horizontal and vertical dividers that separate a window into smaller panes.
H
2 entries
L
5 entries
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Laminated glass
Laminated glass consists of two or more layers of glass bonded together with a tough, transparent interlayer-typically polyvinyl butyral (PVB) or SentryGlas® (ionoplast). If broken, the interlayer holds the glass fragments in place, preventing dangerous shards from falling and providing enhanced security, safety, and sound insulation. In heritage windows, laminated glass is often used in overhead glazing, ground-floor sashes, or doors to meet Part Q (security) and Building Regulations for safety glazing—while maintaining a clear, authentic appearance when paired with traditional putty glazing.
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Lead
Used for centuries as counterweights inside sash box frames, and occasionally as cames in leaded lights — heavy, soft, malleable.
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Lift
A small handle — usually brass or timber — fitted to the bottom rail of a sash to allow it to be raised cleanly.
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Listed building
The United Kingdom protects special architectural and historic structures through formal recognition as Listed buildings which receive legal…
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Low-E glass
Low-E (low-emissivity) glass contains a thin metal coating which allows sunlight to enter, but bounces heat back into the room. The window achieves…
M
2 entries
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Meeting rail
The meeting rail is the horizontal edge where the upper and lower sashes of a double-hung window come together when closed. In traditional timber sash windows, this joint is carefully crafted to ensure a tight seal-often enhanced with modern draught-proofing brushes or seals-while maintaining smooth operation. The meeting rail is also a key location for security hardware and, in fire-rated applications, may incorporate intumescent strips to prevent smoke and flame spread.
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Microporous paint
Microporous paint functions as a high-performing breathable coating which developers designed to protect exterior timber surfaces including windows and doors by enabling water vapor to escape while blocking liquid water penetration. The wood remains protected from moisture buildup because this process enables air circulation which stops moisture accumulation that leads to rot and blistering and peeling. The main difference between conventional paints and microporous paints is that the latter allows wood to move naturally which results in better durability and extended maintenance periods for heritage timber windows used in UK weather conditions.
O
2 entries
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Open-topped pediment
An open-topped pediment is a decorative architectural feature where the traditional triangular or segmental crown above a window or door is left open at the apex, creating a stylised ‘U’ or broken arch effect. Popular in Georgian and Regency design, it adds visual lightness and classical flair without the formality of a full pediment. In sash window surrounds, open-topped pediments are typically crafted from timber and integrated into the architrave-often seen in townhouses and villas seeking elegant yet understated ornamentation. During restoration, preserving this detail is key to maintaining period authenticity.
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Outside lining
An outside lining (also known as an external reveal liner or box frame extension) is a timber component fitted to the exterior of a sash window frame to bridge the gap between the window box and the outer wall surface-especially in older properties with thick brick or stone walls. It ensures a clean, weather-tight transition while maintaining the window’s alignment and historic proportions. In conservation work, outside linings are carefully matched to original profiles and finished with breathable microporous paint to prevent moisture trapping and support long-term durability.
P
4 entries
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Parting bead
A slender vertical strip of timber fixed inside the box frame that separates the running paths of the upper and lower sashes.
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Pavement light
A glazed panel set into a pavement or floor — Victorian iron and glass units that brought daylight into basements and vaults.
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Pediment
A pediment is a decorative triangular or segmental feature crowning a window, door, or portico—commonly seen in Georgian and Regency architecture. In sash window design, pediments add classical elegance, often crafted from timber or moulded plaster and integrated into the architrave surround. While largely ornamental today, they historically signified status and proportion. During heritage restorations, original pediments are carefully preserved or replicated to maintain architectural authenticity, especially in Conservation Areas and Listed Buildings.
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Putty glazing
Putty glazing is the traditional way glass is held into a timber window. The pane sits on a bed of material in the rebate of the sash, is secured…
S
7 entries
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Sash cords
Sash cords are the braided cotton or synthetic ropes traditionally used in counterbalanced sash windows to connect the moving sash to hidden weights…
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Sash horns
Sash horns are the projecting extensions at the top corners of a sash window’s lower sash (and sometimes the upper sash), designed to strengthen the…
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Sash Weights
Sash weights are heavy counterbalances-traditionally made of lead or cast iron-hidden inside the box frame of a sash window. Connected to each sash…
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Scotia moulding
A concave quarter-round trim used in period joinery to soften the transition between surfaces — common at sashes and skirtings.
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Sill
The horizontal timber at the base of a window frame, angled outward to shed rainwater away from the wall below.
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Single-hung window
A vertically sliding window with one fixed upper sash and one operable lower sash — common in early 20th-century British homes.
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Staff Bead
The staff bead is the moulding fixed around the inside face of a box sash frame, holding the lower sash in its channel. It is the innermost bead —…
T
3 entries
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Tabernacle frame
A tabernacle frame is a traditional timber sub-frame used in sash window construction to support the pulley stiles and house the sash weights within the wall reveal. It sits behind the main window box and provides structural stability, allowing the sashes to operate smoothly while keeping the counterweight system concealed. Common in Georgian and Victorian buildings, tabernacle frames are often built into thick masonry walls and are critical to the long-term performance of original sash windows. During restoration, preserving or accurately replicating this hidden component ensures authentic operation and alignment.
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Timber sash windows
Traditional vertically sliding windows made from wood — two counterbalanced sashes gliding within a box frame on cords and weights.
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Top rail
The top rail is the uppermost horizontal member of a window sash, forming the head of the frame that holds the glass panes in place. In sash windows, it’s a critical structural component-often featuring horns in Victorian and Edwardian designs to reinforce the joint with the stiles. During restoration, preserving or accurately replicating the top rail’s profile, thickness, and detailing is essential for both authentic appearance and smooth operation within the box frame.
V
2 entries
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Vacuum Insulated Glazing (VIG)
Vacuum Insulated Glazing (VIG) is an advanced, ultra-thin double-glazing solution ideal for historic and period properties. The VIG system employs two glass panels which maintain a small vacuum space of about 0.2 mm between them through sealed microscopic support pillars. The thermal performance of this design achieves U-values of 0.7 W/m²K within a 6–8 mm profile which enables it to fit original sash window box frames without requiring changes to weights or pulleys or operation systems. The installation process of conservation-friendly upgrades helps protect building authenticity while it improves thermal efficiency and reduces moisture accumulation and sound transmission for heritage structures and protected areas.
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Victorian sash windows
Tall, generously proportioned sashes — typically two-over-two or six-over-six, often with ornate horns and moulded glazing bars.
W
3 entries
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Weight pocket
A weight pocket (also called a sash box or window box) is the enclosed cavity within a sash window frame that houses the counterweights, pulleys, and cords of the traditional weight-and-pulley system. Typically built into the side jambs of the window reveal, it allows the sashes to open and close smoothly while remaining hidden from view. In heritage restorations, preserving or carefully repairing the weight pocket is essential to maintain authentic operation-especially when upgrading to heavier double-glazed sashes, which may require rebalancing with adjusted weights.
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Weight-and-pulley system
A weight-and-pulley system serves as the basic counterbalance mechanism which operates box sash windows. The system contains cast-iron or lead weights which hide inside the box frame while cords or chains connect these weights to the sashes through pulleys positioned at the top of the frame. The weights shift in reverse direction when the sash moves which counterbalances its weight to produce smooth operation and position stability. The historical engineering solution of the past maintains its value because it provides dependable authentic results for heritage window restoration work.
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Window catch
A simple locking or holding device fitted to sash or casement windows to keep them closed and aligned.
Y
1 entry
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Yorkshire light
A Yorkshire light is a traditional type of horizontally pivoting window, commonly found in northern England-particularly in 18th- and 19th-century industrial and rural buildings. Unlike sash or casement windows, it opens by rotating on a central horizontal axle, allowing ventilation from both top and bottom simultaneously. Typically made of timber with simple glazing, Yorkshire lights were prized for their durability and efficient airflow. In restoration projects, they’re often refurbished with draught-proofing pivots and slimline double glazing to improve thermal performance while retaining their distinctive regional character.
A term we missed?
If you've come across a sash-window word that isn't here, send it across — we'll write an entry. The glossary is a working reference, not a fixed list.