The L.S. Starrett No. 20 4.5" Precision Machinist Square is a hardened tool used in machining. It is a precision measuring instrument that is used to check the squareness of surfaces and to set up machines. The tool is made by L.S. Starrett, a well-known manufacturer of precision measuring tools. The square is 4.5 inches in size and is made of hardened steel for durability. It is a must-have tool for machinists who require high precision in their work.