impost

Impost is a term with two principal senses in English. In architecture, an impost is a projecting block or moulding at the top of a column or pier from which an arch rises. The impost marks the transition between the vertical support and the arch and helps to transfer the load of the arch down into the supporting member. In classical and medieval building practice, imposts can be plain or elaborately decorated and may be molded or sculpted to suit the architectural style. The arch itself typically begins at the impost, with the voussoirs forming the curved springing from that point. Imposts thus function as a structural and decorative link between the column or pier and the arch.

Historically, impost also appears as a term for a tax or duty. In older English usage, an impost referred to a levy, especially customs duties or other charges imposed by a government. Today the word in this sense is largely archaic, with modern usage preferring terms such as tax, tariff, or duty. The architectural sense remains common in discussions of historical and contemporary building techniques.

Etymology traces the architectural term to Old French impost or impôt, from Latin imponere, meaning to place upon. The tax sense likewise derives from the same root via French. The two meanings are distinct in modern usage, though they share a common linguistic origin in the idea of placing or imposing something upon a recipient.