A caliper is a type of micrometer designed to make very precise measurements. The addition of a Vernier scale can make the calipers even more precise and easier to read. On a metric caliper, there are ten divisions on the Vernier scale. These divisions represent the second decimal place, 0.01mm. Further subdivisions can be added to maker the measurement even more precise. Having these divisions on a separate scale allows them to be spaced farther apart and, therefore, easier to read.
Set the calipers to the item you wish to measure. Use the thumb wheel on the vernier scale to fine-tune the measurement and then use the locking wheel to lock them in place.
Find the "0" on the Vernier scale and see which number it is pointing to on the fixed scale. If it is between two numbers, select the lower of the two. This is the fixed scale measurement and in the picture, it is 0.4mm.
Find the line on the Vernier scale that precisely coincides with any other line on the fixed scale. The number that it lines up with does not matter as long as it lines up precisely. The number on the line from the Vernier scale is the Vernier measurement. (In the picture it is 0.058). Note that this caliper is accurate to 0.002mm.
Combine the two measurements. In this case, the total measurement would be 0.458mm.
If two lines on the Vernier scale appear to be lined up with two lines on the fixed scale, you can take the average of the two Vernier readings.