For the classic description Ionic capitals by the Roman architect Marcus Vitruvius
Pollio, ca. 90 B.C. to 20 B.C. ,
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Pilaster
An upright rectangular architectural structure that serves as a pier but is treated like a column and usually projects approximately one-third or less of its width from a wall. For additional information and a photo, see the following internet address:
http://architecture.aboout.com/library/blgloss-pilaster.htm