81 Piece Rectangular Steel Gage Block Set Grade AS-2 (B). Comes with NIST Certificate. Grade AS-2 (B) tolerance = ±.000050″. Each block is steel and hardened to 65RC, stress relieved, lapped to accurate size, parallelism & flatness, then carefully inspected and marked for size designed to meet close tolerance. Each set includes standard sizes found in more expensive sets. Each block individually serialized and conforms to the rigid standards of the National Bureau Of Standards. 9 Blocks = .1001-.1009″, Step = .0001″. 49 Blocks = .101-.149″, Step = .001″. 19 Blocks = .05-.95″, Step = .05″. 4 Blocks = 1-4″, Step = 1″.