https://patents.google.com/patent/CN201748877U/en
CN201748877U - A vernier caliper for measuring grooves in deep holes - Google Patents
A vernier caliper for measuring grooves in deep holes, comprising an outer measuring claw (1), a ruler frame (2), a fastening screw (3), a ruler body (4), a...
a vernier
https://lists.ibiblio.org/sympa/arc/machinist/2013-02/msg00020.html
machinist - [machinist] How to Read a Vernier Caliper - YouTube - arc
how to read a vernier calipermachinistyoutubearc