Double Glazing Windows – A Glossary

A window is a window, until you come to purchase new windows and suddenly you’re hearing terms that can mean very little, but can make all the difference to the aesthetics and workings of your windows. Here’s a handy double glazing windows glossary of the industry terms.

Casement: Another name for the sash.

Concealed Drainage: Drains water underneath the window out onto the sill.

Corner Joint: This refers to the finished corner joint. When the joint is welded, it can be grooved or knifed and sits at a 45-degree angle. Timber-look joints are also now available on uPVC windows and are straight to mimic a mechanical joint. See ‘The Emergence of the Mechanical Joint in uPVC Windows’.

Drainage: Our window systems are designed to drain any water ingress through the internal chambers.

Dummy Casement: Used to create an even sightline where non-opening sections are balanced with opening sections. The dummy casement won’t have any window furniture, such as a handle. Also referred to as a dummy opener.

Equal Glass Heights: Equal heights of glass across each section. This is important for a balanced look.

Equal Glass Widths: Equal widths of glass across each section. This is important for a balanced look.

Even Sight Lines: All casements are the same, regardless of whether they open or not.

Externally Glazed: The glass is inserted in to the frame and the glazing beads are on the outside of the frame. Aluminium windows can be externally glazed. The rigidity of the metal beads is less of a security concern than malleable plastic.

Face Drainage: If fitting a window directly onto a stone or tile sill, drainage slots are cut into the bottom section of the outer frame and covered with caps. If the window is being fitted with a purpose-designed sill, this allows for concealed drainage.

Face Fix/Internal Georgian: This is when the Georgian bars are fitted to the outside of the glass on the external side with an 18mm internal bar fitted between the panes. There is no Georgian bar on the interior pane. An easier clean option for inside but still maintaining the authentic Georgian appearance on the outside.

Face-Fix Georgian Bars: Georgian glazing bars fitted to the outside of the glass, both inside and out. This creates a traditional Georgian appearance.

Fanlight: Small opening at the top of the window, hinged at the top.

Fanlight Drop: This is the depth of the fanlight from the top of the outer frame to the centre of transom.

Flush Casement: This is when the casement sits flush within the frame when closed. Read our blog, ‘What Are Flush Casements’ to find out more.

Georgian Bars: These are fitted to the glazing, either internally (see below), externally or both to create the look of individual panes of glass for traditional properties.

Glazing Bead: Holds the glass in position. These are available in two design options; Scotia and Chamfered. Scotia is a traditional ogee profile. Chamfered is a contemporary plain bevel.

Internal Georgian Bars: Georgian glazing bars that are placed between the panes of glass/sealed unit. This makes the window easier to clean. Available in 18mm or 25mm.

Internally Glazed: The glass is inserted in the frame and the glazing beads (which hold the glass in position) are on the inside of the frame. uPVC and timber windows are usually fitted with internal glazing beads for optimum security.

Lead Windows: Lead is used to create the look of individual panes in traditional properties. Options include 6mm, 9mm and 12mm to create a diamond, rectangle or other decorative design. Curved lead is also an option.

Mullion: Vertical division in a window.

Peg Stay: A metal bar used to hold a casement in an open position. This is not required with our hinged options but a dummy peg stay is an option which is fixed to the casement as a traditional design feature.

Sash: The opening part of the window. Also referred to as a casement. Can have various hinged openings, usually top or left, as well as tilt & turn options. Sliding Sash is a different style of window. See here for more.

Sealed Units: Refers to the glazed unit. Two (double glazed) or three (triple glazed) pieces of glass sealed together to create a single sealed unit.

Sill: A ledge forming the bottom part of the window. There are different styles of sills and they can be made from the same material as the window, or a different material, for example, stone or hardwood in a more traditional property.

Transom: Horizontal division in a window.

Trickle Vents: A small opening at the top of a window to allow for secure ventilation. See our FAQ, ‘Are Trickle Vents Necessary?’

If you have any questions or would like to see another term included here, please do let us know.