The A-Z Window & Double-Glazing Glossary

Argon Gas - A safe gas used to fill the airtight gap between double glazed and triple glazed windows. Because argon gas is denser than air, it acts as a superior thermal barrier to minimise heat loss.

Astragal Bars (glazing bars) - Decorative bars fitted to both the external and internal glass panes of a window.

Aperture - The structural opening, hole or gap in the wall where your new window will be installed.

Bay Window - A window that is typically an angular shape. Made up of 3 or more panes, that projects outwards with supporting brickwork below it.

Bottom Rail – The lowest horizontal part of a window sash (the moving section of the window).

Bead – A plastic strip around the edge of the window that holds the glass in.

Casement - One of the most popular styles of uPVC windows and doors. A casement window features a sash attached to the outer frame by window hinges at the top or side, allowing it to swing open outward.

Chamfered – A sharp corner or edge, meaning a chamfered window has straight edges that can look modern and minimalist.

Cill – A window cill (Sill) is the flat horizontal shelf at the bottom of a window on the exterior. Window cill/sills can also refer to the internal called a window board.

Dual Colour – A window that has a different colour/finish on the outside and inside.

Dummy Sash - A non-opening part of a window frame designed to look exactly like an adjacent opening sash. This ensures uniform sightlines across the front of your house.

Double Glazing - A highly energy-efficient window system constructed using two separate glass panes separated by an airtight gap. This gap is sealed with a window spacer and usually filled with argon gas to stop heat transfer.

Espagnolette - A multi-point locking mechanism that runs vertically down the inside of a window frame or door to lock it tightly at multiple points.

Egress/Easyclean Hinges – Specialised window hinges that allow a window to open to a nearly 90 degree angle for a fire exit. These hinges also include a sliding mechanism to allow the outside glass to be cleaned from the inside.

Fixed window – A window that doesn’t have an opening and is ‘fixed’ in place, sometimes called a ‘picture window’.

Floating Mullion – A vertical window or door divider that is attached to one of the sashes. When the window or door is open the view is unobstructed by any frames.

Flush Casement Windows - A fast-growing trend in modern home design. Unlike a standard uPVC casement window where the opening part stands out from the frame, a flush casement sits perfectly flat and level with the frame for a sleek, premium finish.

Gasket - The heavy-duty rubber seal compressed between the window glass and the profile frame to eliminate draughts and prevent rainwater leaks.

Head - A window head is the top horizontal part of the frame.

Integral Blinds (Blinds Inside Double Glazing) - Venetian or pleated blinds permanently sealed inside the airtight gap of a double glazed unit.

Keyed-Alike Cylinder - A security configuration where multiple door or window locks are manufactured to operate using the exact same key.

Laminated Glass (Safety Glass) - A type of safety glass built by sandwiching a tough plastic interlayer between two layers of glass. If impacted, the glass cracks but stays securely bonded to the plastic rather than shattering into dangerous pieces.

Low Emissivity Glass (Low-e) – A type of energy efficient glass that has a transparent coating that reflects internal heat back inside in winter and reflects solar heat out in summer.

Lintel - A load-bearing structural beam installed horizontally directly above a window or door aperture to support the weight of the brickwork above it.

Mechanical Joints - A traditional timber-style frame joint constructed by hand rather than a machine weld. It results in a clean 90-degree butt-joint that perfectly mimics the look of a premium, 19th-century timber window but using modern, low-maintenance materials.

Night Vent – A window night vent refers to a security feature on a window when it can be locked but slightly ajar, with a small air gap to allow for ventilation overnight.

Obscure Window Glass – Textured or frosted glass with reduced transparency, ideal for ensuring privacy in bathrooms or front entrance doors.

Passivhaus - An international, voluntary building standard focused on extreme energy efficiency and thermal performance, minimizing a home's heating needs and environmental footprint.

RAL Colour - The standardized European colour matching system. If you choose custom double glazing window colours, using a RAL code ensures your window frames perfectly match your front door or garage doors.

Sculptured - A window profile shape with a curved, rounded edge.

Secondary Glazing – An independent, slimline glazed panel installed on the inside of an existing window. It is commonly used in listed buildings or conservation areas where replacing the windows is not allowed.

Trickle Vents (Window Ventilation) - Small, adjustable slots built into the top of a window sash or frame. Window trickle vents allow fresh air to naturally circulate into your home even when the windows are locked tight.

Triple Glazing – A highly energy-efficient window system constructed using three separate glass panes separated by an airtight gap. This gap is sealed with a window spacer and usually filled with argon gas to stop heat transfer.

Toughened Glazing - A highly durable form of safety glass. It is heat-treated to be up to five times stronger than standard glass. If it does break, it safely shatters into small, blunt pebbles rather than sharp shards.

U-Values – The scientific measurement used to grade heat transfer through building materials. The lower the U-value, the better insulated the window is.

UPVC - Unplasticised Polyvinyl Chloride. A highly rigid, durable and weather-resistant plastic material that serves as the gold standard for modern window and door frames.

Vertical Sliding Sash Windows - A type of traditional window consisting of two framed panels that slide vertically.

Warm Edge Technology (Spacer Bars) - Modern window spacer bars made from insulated plastic composites rather than old-fashioned conductive aluminium. They keep the edges of the double glazed unit warm, drastically reducing perimeter co

Now that you know your casements from your mullions, you can confidently choose the perfect style for your upcoming home improvements.

If you have any questions about window styles, colours, energy efficiency or free quotes we're here to help.

• Call our friendly sales team on: 01283 212222
• Email us: info@swadlincotewindows.com
• Or come visit us: Unit 10, Boardman Industrial Estate, Boardman Road, Swadlincote, Derbyshire, DE11 9DL