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joint rod, joint rule
A piece of metal, usually 2 to 24 in. (approx. 5 to 60 cm) long and 4 in. (10 cm) wide with a 45° angle cut at one end, used to form and shape mitered plaster joints in cornice work.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.