T-squareT-square (t??skwâr?)n. A rule having a short, sometimes sliding, perpendicular crosspiece at one end, used by drafters for establishing and drawing parallel lines.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.T-square n (Tools) a T-shaped ruler used in mechanical drawing, consisting of a short crosspiece, which slides along the edge of the drawing board, and a long horizontal piece: used for drawing horizontal lines and to support set squares when drawing vertical and inclined lines Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014