/ˈkeɪsmənt/
[noun] A window frame that opens on hinges attached to the side, like a door, rather than sliding up and down. It can also refer to the whole window, especially one that opens outward.
The word 'casement' hides a forgotten architectural term for a hollow moulding — making every romantic poet's window secretly about a groove in stone.
The story of 'casement' begins with the idea of enclosure — of putting something inside a frame or a case. The word traces back to the Old French 'encassement', meaning a frame or a setting, which itself derived from 'encasser', meaning to encase or to fit into a frame. At its deepest root lies the Late Latin 'capsa', meaning a box or a container — the same root that gives us the English word 'case' in its sense of a holder or shell. This Latin 'capsa' came from the verb 'capere', meaning to hold or to take, a verb of remarkable fertility that also produced 'capture', 'capable', and 'capacity'.
When the word entered English in the late 15th century, it did not immediately mean a hinged window. Its earliest recorded use referred to something more architectural and specialised: a hollow moulding or concave groove cut into stonework, the kind of decorative channel you might find running along the edge of a stone frame or column. This is sometimes called a 'casing' even today, and it reflects the original sense of something that encloses or borders another thing. The connection to windows came naturally, since window frames are essentially casings — enclosures that hold glass within a wall.
By the 16th century, 'casement' had settled firmly into its window meaning, referring specifically to a window or part of a window set within a hinged frame. Unlike the sash window, which slides vertically, the casement window swings open like a small door. This style was enormously common in English domestic architecture before the sash window became fashionable in the late 17th century, meaning that for most ordinary English people of the Tudor and Stuart periods, all windows were essentially casement windows — the word named the only kind they knew.
The casement window became a beloved image in English literature, partly because of its physical intimacy. A casement window opens wide, inviting the outside in; you can lean out of it, let in the night air, or listen to the sounds of the garden. Shakespeare used 'casement' several times in his plays, and Keats made the image immortal in his 1819 ode 'The Eve of St Agnes', where he described 'a casement high and triple-arched' through which moonlight poured onto coloured glass. In Romantic poetry, the casement became a symbol of the threshold between the interior world of feeling and the wild exterior world of nature and longing.
It is worth noting that the related word 'case' in the sense of a box or container followed a slightly different path into English: Old French 'casse' → Middle English 'cas' → English 'case'. The word 'encasement' followed the same Old French root as 'casement' but arrived later and more directly. Meanwhile, 'chassis' — the frame of a vehicle — came through French 'châssis' from the same Latin 'capsa', showing how the idea of a containing frame has persisted across centuries and spread into completely different areas of life, from windows to motor cars.