If the world of home improvements is new to you, getting your head around some of the technical terms for windows can be difficult.
To make the process as smooth as possible, we’ve put this helpful list of double glazing terms together for you.

If there’s a term we haven’t covered, don’t hesitate to contact us and we’ll be in touch soon.

large sliding sash

Astragal bars

Astragal bars are set on top of the glazing to create a classic multi-pane window effect.

Bay windows

Typically featuring a central picture window flanked by two angled windows either side, bay windows extend outwards to make rooms feel much more spacious, light, and airy.

Bespoke windows

Windows tailor-made for a particular customer or property style.

Bevelled or chamfered frames

Bevelled window frames (also know as chamfered frames) feature an angled straight edge.

Close up of solid roof
brown flush window selecta

Bottom rail

Sitting above the window cill, the bottom rail is the lowest horizontal bar that interconnects the windows vertical sections.

Casement windows

Typically featuring hinges positioned at the side of the frame to open outwards, casement windows are available in uPVC and aluminium profiles.

small upvc


A horizontal ledge along the outside of a window that directs water away from the wall below.

Double glazing

Two panes of glass separated by an insulating later of argon gas.

double glazing
Angled view of white sliding sash windows

Double hung windows

Windows with an upper and lower sash designed to slide vertically alongside each other, e.g. a double hung sliding sash window.

Fixed light windows

Also known as a picture window, fixed lights cannot be opened.
(the middle section displayed in the accompanying photo)

Aluminium Windows
brown flush sash selecta

Flush sash windows

Window sashes that create a flat exterior surface.

Georgian bars

Georgian bars are set inside the glazing to create an authentic multi-pane window effect.

sliding sash window

Glazing bars

A bar or rigid supporting strip between adjacent panes of glass.


Also known as the Solar Factor or Total Solar Energy Transmittance, the G-value tells you how well window glazing transmits heat from the sun. You can expect more protection against infrared radiation from lower G-values.

sun shining through window
woodgrain flush sash windows

Head jamb

The top horizontal section of a window frame.

Leaded bars

Often arranged in a diamond or square pattern, leaded glazing bars are used to add a traditional decorative finish to windows.

flush selecta
ovolo style window frames

Ovolo frames

Also known as sculptured frames, ovolo frames provide a soft, decorative curved frame perfect for heritage-style properties.

Multi-chambered profile

Multi-chambered uPVC window profiles trap warm air inside, enhancing insulation by preventing cold air moving around the frame.


RAL colour windows

RAL is the most popular European colour matching standard for powder coating, plastics, paints and varnish.


The sash is the moveable part of the window that holds the glazing in place.

Sash horn and hardware

Sash horn

Typically a curved or 's' shape positioned on the top sash, sash horns were originally designed to strengthen sliding sash windows and stop them from opening too far. Today they are added purely for decoration.

Side hung windows

Windows with hinges positioned at the side of the frame that open outwards or inwards, e.g. casement windows.

side hung window

Sliding sash window

Classically styled windows made up of one or two vertically sliding panels, sliding sash windows often feature a heritage 'grid' multi-pane window effect.


The vertical edges of the window.

White flush sash window against a white wall

Tilt & turn window

A modern window style that tilts for safe, secure, and manageable air flow or turns to fully open inwards for easy cleaning and maximum ventilation.

Thermally broken window

Energy efficient plastic barriers positioned between the inner and outer frame of an aluminium window.

Aluminium Windows
An ajar white casement window

Top hung

Windows with hinges positioned at the top of the frame, e.g. an emergency escape window.

Top rail

The horizontal bar that sits underneath the head jamb.

green selecta window

Travel restrictor

Travel restrictors keep children and vulnerable adults safe by limiting how much the sash can open.

Trickle vent

A small opening at the top of the window designed for natural ventilation when closed.

Aluminium Windows


Also known as thermal transmittance, the U-value measures the rate of heat transfer through the entire window (frame + sash + glass). You can expect a more energy efficient home from lower U-values.

Vertical jamb

The vertical sections that form the sides of the window frame.


WER stands for Window Energy Rating and is based on a scale of A+ to G (G being the least efficient rating).

Window hardware

The parts that help the window to operate correctly, e.g. hinges, locks, latches, counterbalances.

Close up of white window hardware
close up of handle

Window furniture

The elements that enhance the appearance of the window, e.g. Monkey Tail handles, Pear Drop handles, D Handles.

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