An impost is that part of an arch from which the arc swings upward. If a capital is the top part of a column, an impost is the bottom part of an arch. An impost is NOT a capital but is often on top of a capital that has no entablature.
An impost needs an arch. An abacus is a projecting block atop a column's capital that does not hold up an arch. The next time you're in Washington, D.C., look up at the columns of the Lincoln Memorial to see an abacus or two.
The Impost Block
Builders of what is now known as Byzantine architecture created decorative stone blocks to transition between columns and arches. Columns were smaller than the thick arches, so impost blocks were tapered, the small end fitting on the column capital and the larger end fitting onto the arch. Other names for impost blocks include dosseret, pulvino, supercapital, chaptrel, and sometimes abacus.
The Look of Imposts
The architectural term "impost" may date back to Medieval times. The interior of the Byzantine-era Basilica of Sant'Apollinare Nuovo in Ravenna, Italy is often cited to illustrate the use of imposts. Built in the early 6th century (c. 500 AD) by the Ostrogoth King Theodoric the Great, this UNESCO Heritage site is a fine example of both mosaics and arches in Early Christian architecture. Note the impost blocks above the capitals of the columns. The arches spring upward from those blocks, which are traditionally highly decorated.
Today's American homes reminiscent of Mediterranean or Spanish architecture will exhibit architectural features of the past. As was typical of imposts hundreds of years ago, the imposts often are painted a decorative color that contrasts with the color of the house itself.
Taken together, these images show the transition of the column (3) to the arch (1) by way of the impost (2).
Origin of the Word
Impost has several meanings, many of which may be more familiar than the architectural definition. In horse racing, "impost" is the weight assigned to a horse in a handicap race. In the world of taxation, an impost is a duty imposed on imported goods - the word is even in the U.S. Constitution as a power given to Congress (see Article I, Section 8). In all of these senses, the word comes from a Latin word impositus meaning to impose a burden onto something. In architecture, the burden is on a part of the arch that holds it up, denying gravity's attempt to bring the weight of the arch to earth.
Additional Definitions of Impost
"The springing point or block of an arch." - G. E. Kidder Smith
"A masonry unit or course, often distinctively profiled, which receives and distributes the thrust of each end of an arch." - Dictionary of Architecture and Construction,
The Impost and Arch in Architectural History
Nobody knows where arches began. They aren't really needed, because the Primitive Hut post and lintel construction works just fine. But there's something beautiful about an arch. Perhaps it's man's imitation of creating a horizon, creating a sun and a moon.
Professor Talbot Hamlin, FAIA, writes that brick arches date back to 4th millennium BC (4000 to 3000 BC) in the region known today as the Middle East. The ancient land called Mesopotamia was partially enveloped by the Eastern Roman Empire during the long period we sometimes call the Byzantine civilization of the Middle Ages. It was a time when traditional building techniques and designs already developed in the Middle East combined with the Classical (Greek and Roman) ideas of the West. Byzantine architects experimented with creating higher and higher domes using pendentives, and they also invented impost blocks to build arches grand enough for the great cathedrals of Early Christian architecture. Ravenna, south of Venice on the Adriatic Sea, was the center of Byzantine architecture in 6th century Italy.
"Later still, it came gradually to replace the capital, and instead of being square at the bottom was made circular, so that the new capital had a continuously changing surface, from the circular bottom on top of the shaft up to a square of much larger size above, which supported the arches directly. This shape could then be carved with surface ornament of leaves or interlacing of any desired intricacy; and, to give this carving greater brilliance, often the stone beneath the surface was deeply cut away, so that sometimes the entire outside face of the capital was quite separate from the solid block behind, and the result had a sparkle and a vividness which was extraordinary." - Talbot Hamlin
In our own homes today we continue the tradition begun thousands of years ago. We often decorate the impost area of an arch if and when it protrudes or is pronounced. The impost and impost block, like many architectural details found on today's homes, are less functional and more ornamental, reminding homeowners of past architectural beauty.
- G. E. Kidder Smith, Source Book of American Architecture, Princeton Architectural Press, 1996, p. 645
- Dictionary of Architecture and Construction, Cyril M. Harris, ed., McGraw-Hill, 1975, p. 261
- Talbot Hamlin, Architecture through the Ages, Putnam, Revised 1953, pp. 13-14, 230-231
- Photo of Lincoln Memorial by Hisham Ibrahim/Getty Images (cropped); Photo of Spanish-style home by David Kozlowski/Moment Mobile Collection/Getty Images (cropped); Photo of colonnade and arches inside the Basilica of Sant'Apollinare Nuovo by CM Dixon Print Collector/Getty Images (cropped); Illustration of an impost by Pearson Scott Foresman Public domain, via Wikimedia Commons