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  • Writer's pictureDavid Taylor

Understanding Windows

When you are considering purchasing new windows, it is very useful to have a basic understanding of the conventions which exist within the fenestration industry, the terminology used and what the window drawings within a quotation are telling you.


As I have mentioned in a previous post, when you see a windows dimensions as being 1800 x 1200, this means that the width of the window is 1800mm and its height is 1200mm. In other words, the width measurement is always written first. The same convention is used for all doors - Entrance, Bi-fold and Sliding. In the UK we always use millimeters and as a client, it is best to do the same. Whilst converting centimeters and inches into millimeters is not difficult, doing the conversion can lead to errors being made. Just ask NASA about the incident in 1999 when it lost the Mars Climate Orbiter. Granted, the cost of the Orbiter ($125 million) is somewhat more than that of a few windows, but I think you probably get my point. The moral of the story is, measure in millimeters and stick with millimeters!

It is also fenestration industry practice to draw all window (and doors) viewed from the outside, i.e. looking into property. This is not so important when a window has symmetrical layout, e.g. an opening casement at each end with a fixed pane in the middle (as the window will look very similar from either side), however, is critical when for example there is only one opener on only one side of the window. As I'm sure you can visualise, from the outside the opener may be on the left, although from the inside it will be on the right. A very simple concept to understand, however you will be surprised how many stories I have heard (not from AluFen I hasten to add!) of windows and doors arriving on site not looking or functioning quite how the client expected.


There are various terms and phrases used within the fenestration industry which, although you probably do not need to know all of them, it is useful to know a few of the main ones to enable you to explain what you want and question your supplier using the correct 'language'. This is probably best done by use of the diagram below.

Names of parts of a casement window
The terminology used for a basic casement window

In case you were wondering, the lintel is not part of the window. The lintel (usually made of steel, but can also be made of prestressed reinforced concrete or timber) is an integral part of the building. Its purpose being to support whatever is above the window or door. All windows and doors - whether aluminium, uPVC or timber - are not designed to support the weight of whatever is above them and will not function properly or fail if they are asked to do so.

You can see in the diagram above the 'arrowheads' in the casement and the top light. These show that both elements open, but as importantly, how they open. The mantra being that 'the arrowhead points to the hinges'. Bearing this in mind, you can easily see that the casement has its hinges on the left (so it is referred to as side-hung), where as the top light has its hinges at the top (top-hung).

This only leaves the direction in which the openers open, i.e. open in or open out. The list below is the most common way in which fenestration in the UK function:

Windows: Open out, however bottom hung (hinges at the bottom) casements open in

Front Doors: Open in

Back and Side Doors: Open in

Bi-fold Doors: Open out

Sliding Doors: N/A

Whist the list above are the most common ways in which fenestration open in the UK, there are exceptions to these common rules, therefore if you are in doubt, ask your supplier or make it clear on any information you provide to them how you would like each opener to work, e.g. open-in or open-out.

There is also some useful information and terminology to know about the glazed units (sealed units) which are installed within the window frames and casements, however I will leave that to a separate post.

I hope you have found this post useful.

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