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Bay Windows in Architecture: Definition, Types & Origin

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Some windows are focused on letting in light. Others are built to change the exterior appearance of a structure. Bay windows do both. In this lesson, we'll check out the history and design of bay windows, and see how to use them.

Bay Windows

Families can be like oceans. They're deep and full of life, but they can also make you feel like you're drowning or surrounded by sharks. Sometimes, you need a chance to escape for just a second to a something calmer than the ocean, a sheltered bay, perhaps. Well, your home may be designed for just that purpose. A bay window is a glass-sided recess in the home that protrudes outwards from the house. From the exterior, it looks like a decorative expansion growing out of the house. From the interior, however, it's the equivalent of what a bay is to the ocean: a small inlet that's slightly separated from main space. Plus, if you need to take a five-minute break from your family, it can be just the sheltered harbor you're looking for.

A row of bay windows
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Function

Unfortunately, the formal purpose of a bay window is not to create a little space to sit in solitude and hide from relatives. It's to increase the surface area of a wall, and therefore increase the amount of natural light that can enter through that wall. Basically, by extending the window beyond the wall, you let more light in than if you had set the window into the flat wall itself. That's the intended function of a bay window. Many people also like this feature, however, because it adds a bit more space to the interior as well, making a room look larger or more spacious.

A bay window as seen from inside the house
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History

The bay window is a prominent part of many modern homes, but where did this idea come from? Believe it or not, the light and airy bay window we know and love today may have originated in the heavy, serious stone cathedrals of the Gothic era. As these churches grew in size, small chapels or apses were added as private worship spaces for donors, protruding from the building. Windows were added into these projecting rooms for lighting, and the ancestor of the bay window was born.

Bay windows became more popular in domestic architecture during the Renaissance, but first really took off right after that in the Baroque era. Baroque architecture is focused on dramatic lighting and tons of ornate and decorative additions. The bay windows, projecting from the wall, fulfilled both of these, bringing more light into the interior while also breaking up the exterior façade with angled configurations of glass.

Styles

So, what does the bay window look like today? There are many ways to build a bay window, most of which involve setting 3 or more panes of glass into a specific shape. It is the shape of the bay window which most characterizes its style, although its placement on the house is also important.

The box bay window is shaped like a box, generally a rectangular one. With flat sides, a flat front, and some sort of roof, this is an extremely common style of bay window that can be found across Europe and the Europeanized Americas. If the sides of the box bay window are angled instead of flat, it may also be called a canted window.

Box bay windows
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But what if you don't want such a rectilinear shape? There are two styles of bay windows that are based on curves. A gently curved, semi-circular configuration of glass panes creates a bow bay window, sometimes just called a bow window. These became popular in the country estates of 18th-century England, and in the late colonial/early republic United States as well. In general, you need at least four segments of glass panes to create a bay of this shape.

Bow windows
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The bow bay is a gently arching feature, but you can tighten that into a more extreme visual and decorative element called the circle bay window. The circle bay, like the bow bay, is semicircular but more visually prominent. As a result, they're very often embellished with crown moldings or other decorations, but are also very functional in bringing additional natural light into a room.

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