The vernier caliper was invented by Pierre Vernier in 1631.
 It has been used until modern times.
 A vernier caliper is a tool for measuring finer dimensions than a regular ruler.
 It can measure up to 36mm with 0.1mm resolution.
 You can measure the outside (sheet thickness), inside (gap width), and depth (step).
 For portability and safety, the blade for inner measurements is foldable.