a device for determining perpendicularity, consisting of a narrow board with a plumb line and bob suspended from an upper edge.
Origin of plumb rule
1350–1400; Middle English plomreule
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a plumb line attached to a narrow board, used by builders, surveyors, etc
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012