An oriel window is a set of windows, arranged together in a bay, that protrudes from the face of a building on an upper floor and is braced underneath by a bracket or corbel. Most people call them "bay windows" when located on the first floor and "oriel windows" only if they are on an upper floor.
Functionally, oriel windows not only increase the light and air entering a room, but also expand the floor space without changing the building's foundation dimensions. Aesthetically, oriel windows became a landmark detail for Victorian-era architecture, although they are present in structures earlier than the 19th century.
Origin of the Oriel:
This type of bay window probably originated during the Middle Ages, in both Europe and the Middle East. The oriel window may have developed from a form of porch-oriolum is the Medieval Latin word for porch or gallery.
In Islamic architecture, the mashrabiya (also called moucharabieh and musharabie) is considered a type of oriel window. Known for its ornamented lattice screen, the mashrabiya traditionally was a protruding box-like architectural detail that functioned as a way to keep drinking water cool and interior spaces well-ventilated in a hot Arabian climate. The mashrabiya continues to be a common feature of modern Arab architecture.
In Western architecture these protruding windows most certainly attempted to catch the movement of the sun, especially during winter months when daylight is limited. In Medieval times, capturing light and bringing fresh air into interior spaces was thought to benefit health, both physically and mentally. Bay windows also expand the interior living space without changing the footprint of a building-a centuries-old trick when property taxes are calculated on a foundation's width and length.
Oriel windows are not dormers, because the protrusion does not break the line of the roof. However, some architects such as Paul Williams (1894-1980) have used both oriel and dormer windows on one house to create an interesting and complementary effect (view image).
Oriel Windows in American Architectural Periods:
The reign of British Queen Victoria, between 1837 and 1901, was a long era of growth and expansion in both Great Britain and the United States. Many architectural styles are associated with this time period, and particular styles of American Victorian architecture are characterized by having protruding window sets, including oriel windows. Buildings in the Gothic Revival and Tudor styles often have oriel windows. Eastlake Victorian, Chateauesque, and Queen Anne styles may combine oriel-like windows with turrets, which are characteristic of those styles. Many urban brownstone facades in the Richardsonian Romanesque style have oriel windows.
In American skyscraper history, the Chicago School architects are known to have experimented with oriel designs in the 19th century. Most notably, John Wellborn Root's spiral staircase for the 1888 Rookery Building in Chicago is known as the oriel staircase. Root's design is actually a fire escape required by the city after the Great Chicago Fire of 1871. Root enclosed the stairs in what architecturally appeared to be a very long oriel window attached to the rear of the building. Like a typical oriel window, the staircase did not reach the ground floor, but ended on the second floor, now part of the elaborate lobby design by Frank Lloyd Wright.
Other architects in 19th century America used oriel-like architecture to increase interior floor space and optimize natural light and ventilation in "the tall building," a new form of architecture that would become known as the skyscraper. For example, the architecture team of Holabird & Roche designed the 1894 Old Colony Building, an early Chicago School tall building, with all four corners protruding. The oriel towers start on the third floor and hang over the lot line or footprint of the building. The architects had cleverly found a way to use airspace to increase square footage beyond the property line.
Summary of Characteristics:
Oriel windows have no strict or definitive definitions, so know how your locality defines this architectural construction, especially when you live in a historic district. The most obvious identifying characteristics are these: (1) As a bay-type window, the oriel window projects from the wall on an upper floor and does not extend to the ground; (2) In Medieval times, the bay was supported by brackets or corbels underneath the protruding structure-often these brackets were highly ornate, symbolic, and even sculptural. Today's oriel windows may be engineered differently, yet the bracket remains-traditional, but more ornamental than structural.
One could even argue that the oriel window is forerunner to Frank Lloyd Wright's cantilever construction.