This idea comes from two pieces of prior art: the Vernier scale, most commonly used on calipers, and dials for displaying all sorts of numbers, including on calipers.

But no one seems to have ever combined them.

Let's consider your basic dial scale caliper. As you move the part with the dial along
the scale, you read the main part of your measurement (centimeters, for example) off the straight part of the tool, then the smaller part (millimeters, in this case) off the dial. For simplicity, the dial has 10 evenly spaced marks and a needle. Each mark is a thin line going from the center, almost to the edge, with a digit 0 through 9 next to it. It's trivial to see how to use this; just add the centimeters and millimeters to get your measurement.

Now, let's change it up a bit: Replace the needle with a clear plastic disk, with eleven evenly spaced marks. The first mark is a thin red line extending all the way to the edge. The other ten marks are thin blue lines which extend from the middle, and are slightly shorter than the lines of the lower dial. The blue lines are numbered one through ten. The red line is the zero.

Reading this is a bit more complicated, but quite worthwhile.

The red line points at or past the number of millimeters, exactly like the needle of the original dial.

Then, one identifies which of the red or blue lines on the clear disk is most closely aligned with the black lines on the lower disk. This number is one tenth of a millimeter. Due to the moiré effect, this should be easy to identify... the lines closest to the overlapping ones will look "lighter" and the lines furthest will look "darker".

Now you're thinking, what's the point, you've only gained one digit of precision. Actually, I've double the precision of the dial.

If the lower dial has 100 marks instead of 10, and the upper (clear) disk has 101 marks instead of 11, we can make measurements of one 1000th of a millimeter.

That's a lot of extra precision for the cost of using a clear disk instead of a needle, and a bit of extra ink!

I leave the task of making the tool more accurate, as well as precise, to other great thinkers.