What is an impost? What is an impost block?

The Base of the Arch

Collage of three photos explaining the word IMPOST
1. End of arch; 2. Impost; 3. End of column (view larger image). Photo (left) by CM Dixon/Print Collector/Hulton Archive Collection/Getty Images (cropped); llustration by Pearson Scott Foresman [Public domain], via Wikimedia Commons; Photo (right) by David Kozlowski/Moment Mobile Collection/Getty Images (cropped)

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 often on top of a capital that has no entablature.

Images of Imposts:

The collage on this page (view larger image) illustrates the architectural term "impost." The photo on the left is the interior of the Byzantine-era Basilica of Sant'Apollinare Nuovo in Ravenna, Italy.

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.

The center illustration shows the transition from the column (3) to the arch (1) by way of the impost (2).

The photo on the right shows a modern American home reminiscent of Mediterranean or Spanish architecture. As was typical of imposts hundreds of years ago, the imposts on this home are painted a decorative color that contrasts with the color of the house itself.

Additional Definitions of Impost:

"The springing point or block of an arch."—G. E. Kidder Smith, Source Book of American Architecture, Princeton Architectural Press, 1996, p. 645
"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, Cyril M. Harris, ed., McGraw- Hill, 1975, p. 261

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
  • abacus

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 US 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.

The 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 what we call today 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 Byzantine architecture combined the Classical ideas of the West with building techniques and designs already developed in the Middle East. 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 Italy. 

About the Byzantine Impost Blocks of the 6th Century:

"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, Architecture through the Ages, Putnam, Revised 1953, pp. 230-231

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.

Source: Talbot Hamlin, Architecture through the Ages, Putnam, Revised 1953, pp. 13-14