The jet black straight grooves on both sides of the master bar  allow for positive alignment of graduations and flush fitting of the vernier  scales to the main scale, which eliminates parallax. 
                                              Can measure outside diameter (O.D), inside diameter (I.D), depth  and steps 
                                              Stainless steel, hardened throughout. Satin chrome finished vernier and main  scale with jet black graduations for 
                                              glare free reading. All graduations are marked by laser machine for  accuracy. 
                                              With depth measuring  bar | 
                                           
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