reference (AAA): small tolerance (±0.05 μm) used to establish standards
calibration (AA): (tolerance +0.10 μm to −0.05 μm) used to calibrate inspection blocks and very high precision gauging
inspection (A): (tolerance +0.15 μm to −0.05 μm) used as toolroom standards for setting other gauging tools
workshop (B): large tolerance (tolerance +0.25 μm to −0.15 μm) used as shop standards for precision measurement
More recent grade designations include (U.S. Federal Specification GGG-G-15C):
0.5 — generally equivalent to grade AAA
1 — generally equivalent to grade AA
2 — generally equivalent to grade A+
3 — compromise grade between A and B