The two large marble altars stand aligned in an east-west direction in the central area of the Agora . One, called Altar A, stands almost in the centre of the Lower Platea, facing the overlooking North Portico; the other, called Altar B, flanks the first to the east. The two altars are the same in their external structure, but differ in their general dimensions, the cut and thickness of the blocks, the finishes and the unit of measurement used. The elevation consists of a high crest of four steps enclosed by parapets, the last of which, wider than the others, serves as a prothysis, and the altar case, formed by a crowning cornice.