Why is the ninth month called September?
triangular part of the facade of a Greek-style building, 1660s, alteration of periment, peremint (1590s), of unknown origin, "said to be a workmen's term" [OED]; probably a dialectal garbling of pyramid, the connection perhaps being the triangular shape. Sometimes associated with ped- "foot." Other possibilities include Latin pedamentum "vine-stalk, prop," and Italian pedamento, which at the time this word entered English meant "foundation, basework, footing." Meaning "base, foundation" is from 1726, by inflience of Latin pedem "foot."
A broad, gently sloping rock surface at the base of a steeper slope such as a mountain, often covered with alluvium. Pediments are formed through the exposure of bedrock by erosional processes, such as the flow of water. Pediments are usually found in arid regions where there is little vegetation to hold the overlying soil.