From Casements to Mullions: A Handy Design Glossary for Windows

From Casements to Mullions: A Handy Design Glossary for Windows

Anna Hoffman
Feb 3, 2012

Readers, I know you're shy, so I've decided to be proactive and answer the burning window-vocabulary questions I know you've been quietly ruminating over for years. What's the difference between a mullion and a came, an oriel and a bay? Are your windows sash or casement? Are a transom and a clerestory the same thing? What did Edith Wharton have against picture windows? Read on and find out!

We looked at the history and development of window glass last week, so we know about how pre-Modern people needed to balance their need for indoor light with their need for warmth. Medieval windows (image 2) were typically very small, since they weren't glazed (didn't have glass in them), and they also had mullions, or vertical supports, to maintain the integrity of the structure (a minor detail). Many early homes also featured eyebrow windows, or little curved openings in the roof for light and for ventilation (image 3). Going back to ancient times, churches and other tall buildings had clerestory windows (image 4), which provided light close to the ceiling and helped illuminate large interiors. (More modern transom windows (image 5) are a subset of clerestory windows — they are windows right above doors.)

During the 16th century, window glass became more and more common, leading to the development of different window types (image 6). Once people started glazing their windows or covering them, they developed casement windows (image 1) that opened like little doors. These would often be doubled by wooden shutters that would open the same way. In the 17th century, the best homes started installing sash windows, where two parts of a window would slide past each other and then be fastened in place to stay open (image 7). By the end of the century, special counterweights and catches were designed to make these more convenient to use.

It was also in the 17th century that so-called French windows were invented — these are simply casement windows that go all the way to the floor. Madame Rambouillet, the famous society hostess, was supposedly the first to install them. It was also during the 1600s that the architect Francois Mansart popularized sloped roofs (now known as Mansard roofs) and started putting dormer windows in them so that the attics could be used for bedrooms (image 3).

Oriel windows were originally found in churches and castles, like a partially glassed-in turret (image 8). A window that projects out from the upper stories of a building, oriels are the ancestor of bay and bow windows. There is also a definite echo between oriels and mashrabiyas, which are upper-story wooden-shuttered bays in Islamic architecture (image 9). There are subtle distinctions between oriels, bay windows and bow windows. Bay windows are usually found on lower floors, and generally have a large center panel and two smaller side panels that are angled out towards the center. A bow window might have more panels, and it forms more of a rounded bow shape. In 18th century England, there was a big vogue for Palladian windows, based on the principles set forth by Renaissance architect Palladio, which featured a large central arch window flanked by two smalled rectangular windows, like a bay window but flat (image 10). These are still favorites in Georgian style homes.

Once plate glass glass came along, picture windows became popular, plain panels of glass with no mullions or muntins to impede your view. But while these have their place, people are still drawn to the more traditional window forms. In their famous 1897 book, The Decoration of Houses, Edith Wharton and Ogden Codman, Jr., explained why it isn't always a good idea to go for the full picture window:

Where there is a fine prospect, windows made of a single plate of glass are often preferred; but it must be remembered that the subdivisions of a sash, while obstructing the view, serve to establish a relation between the inside of the house and the landscape, making the latter what, as seen from a room, it logically ought to be: a part of the wall-decoration, in the sense of being subordinated to the same general lines. A large unbroken sheet of plate-glass interrupts the decorative scheme of the room ….

Are there any other windows that you want the dirt on? Any other burning questions you might have?

Images: 1 Casement windows in Paris via Kitchen Thymes; 2 Giotto's Campanile via; 3 Dormer and Eyebrow Windows via; 4 Clerestory Windows via Alicia B designs; 5 Transom window photo by Frederic Hooft via Desire to Inspire; 6 Hampton Court Tudor window via BuilderBill; 7 Window via OnlineFurnitureDesign; 8 Oriel Window in the Abbot's Porch (1509) at Cerne Abbey, via Sacred Destinations; 9 Mashrabiya via Totems City; 10 Palladian window at Chiswick House via The Brimstone Butterfly.

Sources: and The Window Glossary were both very helpful.

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