The anatomy of a casement window: explained

3 min read Windows

It's all very well referring to a window as 'a window' but if you need to repair or replace them it's handy to know the names of each part of the window. So, to help you out, we’ve listed the whole anatomy of the popular casement window style for absolute clarity.

Casement window terminology explained


A structural horizontal beam over the top of a window, generally used for load-bearing and decorative purposes. Typically found in homes built with brick or masonry blocks, lintels are usually made from concrete, timber, or steel.

Head jamb

The head, or head jamb, is the uppermost horizontal section of the window frame.

Top rail

The horizontal section that sits below the head jamb.

Vertical jamb

Jambs, or vertical jambs, are the outermost sections that form the vertical sides of the window frame.

Window frame

The window frame is a collective term that covers the head jamb, the vertical jambs, and the cill.

Light / pane

Panes, also known as lights, are the glass areas of a window.


A transom is a horizontal beam that is paired with a mullion that divides a window’s upper part into additional lights. When transoms are positioned, generally they will be placed above head height to avoid obstructing the view from a window.

window transom flush timber casements


A window mullion is a vertical beam that divides a window into smaller glass units. The purpose of a mullion on a window is to provide support to the glazing of a window, but they may also be required to provide structural support to a window arch. When used to support glazing, window mullions are often paired with transoms for extra support.


The vertical edges of a casement window.

Bottom rail

The larger horizontal section that sits above the sill.

Sill / cill

The bottom-most horizontal section of the window that often projects outwards.


The frame that holds the glazing in place.


A sash that's attached to the frame by one of more hinges, typically opening outwards from the side of the frame.

Fanlight / Top light

A fanlight is the small top opening in a window unit. Hinged to a transom, fanlight windows enable safe ventilation of interior spaces.

Fixed light

A window that cannot be opened. Used to enhance far-reaching views, popular casement window arrangements often feature a centre fixed light section.

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