Here is an actual vernier scale as used on a measuring instrument. It allows the measuring scale on the left side to be made much more accurate. Instead of reading 756.0 as shown in the photo it could also measure 756.1 by moving the vernier scale up a tiny bit until the second line on the vernier scale was perfectly inline with the mark at 757. At that point you would read it as 756 + 0.1 or 756.1 (0.1 from the vernier scale).
Before digital electronics invaded the machine shop measurement field it was a basic skill that had to be mastered. I still have many precision tools that use a vernier to get that last "digit". I find them very accurate and they can sit in a drawer for years without worry of a battery leaking.
BillWojo