a tool with two blades that are at a fixed angle to one another, used to bevel a mitre joint
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
Examples from the Web for mitre square
When permanently fixed at an angle of 45, it is called a mitre-square.
By placing this bevel against the edge a try-square of this construction can also be used as a mitre-square (Fig. 682).