Machinists Square, 4" 

4" X 3" Machinist square made of hardened tool steel.

Length of blade is measured from inner edge of beam to end of blade. The small notch at the inside corner prevents small particles from accumulating at the juncture and impacting the squares reading.

Typical use involves aligning the beam along one edge of an object with the blade across the body of the work. Using a strong light source behind the square will reveal any gaps showing the squareness of the work.