"Compound pier or cluster pier is the architectural term given to a clustered column or pier which consists of a centre mass or newel, to which ...
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engaged or semi-detached shafts have been attached, in order to perform (or to suggest the performance of) certain definite structural objects, such as to carry arches of additional orders, or to support the transverse or diagonal ribs of a vault, or the tie-beam of an important roof. In these cases, though performing different functions, the drums of the pier are often cut out of one stone. There are, however, cases where the shafts are detached from the pier and coupled to it by annulets at regular heights, as in the Early English period." - (en.wikipedia.org 21.09.2020)
