Matched 1-2-3 Block Set Features include... Matched Pair Precision ground and case hardened on all six surfaces. Squareness of all sides to 0.0001" 11 holes on three faces. Five tapped holes (5/16" - 18) arranged for holding down and conversion to an angle plate. Surface finish 6 to 16 micro inches. Blocks are over sized to allow for normal lapping to 0.0003".