An impost block is the square or polygonal block placed between a column capital and an arch in Byzantine architecture.
The impost block, also called an impost, helped adapt the horizontal top of a capital to the springing point of an arch. By widening the support above the capital, builders could distribute the arch’s load more effectively and create a visually continuous transition between column and vault.
Byzantine imposts often have a trapezoidal or cushion-like form and may carry carved crosses, foliage, monograms, or geometric decoration. They became especially prominent in late antique and early Byzantine buildings, where arches and domes increasingly replaced classical trabeated forms.
An impost block is not an archivolt. An archivolt is the ornamental molding that follows the curve of an arch. A drum is the cylindrical or polygonal portion of a building that supports a dome, while a cornice is a projecting horizontal architectural member.