Abacus - Part of a column known as an abacus
Abacus is a part of a Greek column.
Photos from out of copyright architecture textbooks depicting the columns, representative temples, and entablatures of three orders: Doric, Ionic, and Corinthian.
Vitruvius on Architecture Book IV
From Lacus Curtius, an English translation of the writing on temple architecture by the first century B.C. architect, Vitruvius.
How to Tell Columns Apart
From the About Guide to Greece for Visitors, a short, easy to follow guide to distinguishing one column from the next -- even if the column is broken.
Vitruvius on the ratio of diameter to height and other standard features of Doric columns.