Roughly triangular area on either side of an arch, bounded by a line running horizontally through its apex, a line rising vertically from the springing of the arch, and the exterior curve of the arch. When arches adjoin, the entire area between their crowns and springing line is a spandrel. If filled in, as is ordinarily the case, the result is a spandrel wall; in medieval architecture this was usually ornamented. In buildings of more than one story, the spandrel is the area between the sill of a window and the head of the window below it. In steel or reinforced-concrete structures, a deep spandrel beam may span across this area. The triangular area of space beneath a stair is also known as a spandrel.
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A spandrel (less often spandril or splaundrel) is the space between two arches or between an arch and a rectangular enclosure.
There are four or five accepted and cognate meanings of spandrel in architectural and art history, mostly relating to the space between a curved figure and a rectangular boundary - such as the space between the curve of an arch and a rectilinear bounding moulding, or the wallspace bounded by adjacent arches in an arcade and the stringcourse or moulding above them, or the space between the central medallion of a carpet and its rectangular corners, or the space between the circular face of a clock and the corners of the square revealed by its hood. Also included is the space under a flight of stairs, if it is not occupied by another flight of stairs. This is a common location to find storage space in residential structures.
In a building with more than one floor the term spandrel is also used to indicate the space between the top of the window in one story and the sill of the window in the story above. The term is typically employed when there is a sculpted panel or other decorative element in this space, or when the space between the windows is filled with opaque or translucent glass, in this case called spandrel glass.
The spandrels over doorways in Perpendicular work are generally richly decorated. At Magdalen College, Oxford is one which is perforated. The spandrel of doors is sometimes ornamented in the Decorated period, but seldom forms part of the composition of the doorway itself, being generally over the label.
Because arches are commonly used in bridge construction, spandrels may also appear in those structures. Historically, most arch spans were solid-spandrel, meaning that the areas between arches were completely filled in — usually with stone — until the advent of steel and reinforced concrete in the 19th and 20th centuries. Open-spandrel bridges later became fairly common, where thin ribs were used to connect the upper deck to the bridge arches, resulting in a significant savings in material, weight, and therefore cost. Zhaozhou Bridge in China is the oldest example (built 595-605). Reinforced-concrete open-spandrel bridges were fairly common for crossing large distances in the 1920s and 1930s.
Spandrels can also occur in the construction of domes, and are typical in grand architecture from the mediaeval period onwards. Where a dome needed to rest on a square or rectangular base, the dome was raised above the level of the supporting pillars, with three-dimensional spandrels called pendentives taking the weight of the dome and concentrating it onto the pillars.