having an equilateral triangular cross section, as certain files.
Origin of three-square
1400–50; late Middle English thre square threefold, modeled on foursquare
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for three-square
Historical Examples of three-square
Three-square files are made with equilateral triangular sections, as in Fig. 2223.
The triangular or three-square form is such a form also as is seldom chosen by any that may make another choise.The plant-lore and garden-craft of Shakespeare
Henry Nicholson Ellacombe
having a cross section that is an equilateral trianglea three-square file
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