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Parts of a window

Posted by Adrian
August 27th, 2013

Parts of a window

You will certainly have windows in your property but there are lots of types of window styles such as, a sash window where the top and bottom windows slide up and down, or a casement window, this type of window opens outwards like a door and is very common in new houses.

Windows are also available in many types of materials such as hardwood, softwood, aluminium and Upvc to mention few. The glass may be plan or have coloured glass, lead lights or latticework to make a window more ornate.

Window frame

In this post I am talking about a traditional casement window with a fanlight, however many of the terms used relate to all styles of window.

Window frame

The window frame consists of a head that runs along the top, a sill along the bottom and two jambs running vertically at either side. The frame holds the window itself either hinged as in the case of a casement window or free running as in a sash window.

Top drip

Some windows will have a top drip, this encourages the water to run off the tip of the window rather than to run down the window and glass, or even into an open window.


The casement is the part the glass sits in, the part that opens. A basic design of a casement consists of four parts, a top rail running horizontally at the top, a bottom rail that runs horizontally along the bottom and two styles that run vertically that go to make the casement. This would typically be hinged on one side or along the top and attached to the window frame.


A mullion is the central vertical part that makes up the frame on a casement window and separates two windows, for example a opening casement window and a fixed pain window.


A transom is a horizontal part separating two windows, for example a fixed pain and a fanlight / vent window.

Window sill

The sill is at the bottom of the frame and attached to the two vertical jambs to complete the window frame, the sill will slant away from the window to encourage water to run away from the window frame, the sill will also have grove underneath it to allow the water to drip off and to stop the water running back under the window frame.

Many new windows now come with trickle vents, these are air vents that are in the head of the frame that have a grill on the outside and a closing vent on the inside, this allows air flow into the property and help prevent condensation forming on the windows.

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