A window is a window, until you start shopping for new ones. Then suddenly, you’re faced with terminology that might seem insignificant but can make all the difference to the performance of your windows. To make things a little easier we've collected some of the most common terms together and provided an explanation for each.
Below is our handy A-Z glossary of window and double glazing terminology. This guide will help you sift through industry jargon so you can make an informed, confident decision about your purchase.
Glossary
Astragal Bars - These are decorative glazing bars that are fitted to the external and internal glass panes of a window.
Argon Gas – A safe nontoxic gas which fills the space in between a double-glazed unit. Argon gas is denser than air making it a better insulator.
Aperture – An opening, hole, or gap.
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 lower horizontal bar on a window sash (opening part).
Bead – A plastic strip around the edge of the window that holds the glass in. Typically on the inside (internally beaded) but older windows may be externally beaded.
Casement – A standard style of UPVC window that is attached to its frame by hinges typically at the top or the side.
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 external. Window cill/sills can also refer to the internal which we’d refer to as a window board.
Dual Colour – A window that has a different colour/finish on the outside and inside.
Dummy Sash – This is where a non-opening part of a window is designed to look the same as the opening part of a window. This could be to create uniform sightlines and a balanced look.
Double Glazing – Where a window is made up of two separate panes of glass, separated by an airtight gap with a spacer bar.
Espagnolette - A multipoint locking system, typically mounted on the vertical frame of a window or door.
Egress/Easyclean Hinges – These are specialised window hinges that allow a window to open to a nearly 90 degree angle for a clear 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 – A type of UPVC casement window that sits ‘flush’ within the window frame, providing a smooth, flat look.
Gasket – A rubber or uPVC seal fitted between the glass and frame to prevent draughts and leaks.
Georgian Bar – Decorative bars typically in a horizontal and vertical pattern that are fitted on the inside of double glazing.
Horns (sash) – Sash horns are a decorative feature typically seen on sliding sash windows but can also feature on upvc windows. They appear at the bottom corners of the upper sash.
Head – A window head is the top horizontal part of the frame.
Integral Blinds – These are blinds which are installed in between the two panes of double glazing keeping them protected from dust, dirt and damage.
Keyed-alike Cylinder – Two or more lock barrels configured to operate with the same key, meaning one key fits all.
Laminate Glass – A type of safety glass created by bonding two layers of glass with a plastic interlayer.
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 window lintel is a horizontal structural beam that is situated above a window to support the weight of the wall above it.
Mullion – A window mullion is the vertical part of window that divides it into smaller sections.
Mechanical Joints – A type of window joint that is manufactured by hand. The finish is a butt-joint that replicates a traditional 19th Century timber window.
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 Glass – A type of glass with reduced transparency, available in different levels.
Projecting Cill – A cill at the bottom of a window that extends out from the wall.
Passivhaus – A voluntary building performance standard for high energy efficiency and thermal comfort that reduces a building's carbon footprint.
RAL Colour – A colour matching system that defines colours for paints, coatings and plastics.
Reveal – A window reveal is the frame or surround that finishes the internal edge of a window opening.
Sculptured - A window profile shape with a curved, rounded edge.
Sash – A window sash is the moveable, glazed panel that sits within the frame.
Secondary Glazing – A type of window that sits internally in front of an existing window. It creates an insulating gap to improve thermal efficiency.
Trickle Vents – These are small adjustable ventilation slots that are fitted within a window sash or frame to allow fresh air to circulate.
Triple Glazing – A window that is made up of three separate panes of glass, separated by an airtight gap with a spacer bar.
Toughened Glazing – Safety glass heat-treated to five times the strength of standard panes; designed to shatter into blunt pieces if broken.
U-Values – Measures the rate of heat transfer through a building element.
UPVC – Unplasticised Polyvinyl Chloride, a rigid durable plastic used for window and door frames.
Vertical Sliding Sash – A type of traditional window consisting of two framed panels that slide vertically.
Warm Edge Technology - A type of spacer bar used between panes of glass in double or triple glazing. It’s made from non‑conductive materials to reduce heat loss compared to traditional aluminium spacers.
Welded Joint – A modern diagonal weld that has a clean finish to produce a seam, both internally and externally.
There you have it, your handy A-Z glossary of window and double glazing terminology. Ready to make your purchase? Browse our range of windows here.
If you still have any unanswered questions please speak to our friendly sales team on 0333 004 1100. Or drop them an email to sales@supaglazingonline.com
