Flat arris

Flat arris

The flat edge on a column where a fillet joins the concave surface of a flute
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved

flat arris

An arris (i.e., the external angular intersection of two surfaces), such as between two flutes of a Doric column, which has been flattened so that it is not sharply defined.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.