Vernier Calipers Demonstration

Here's a step-by-step guide to read it:

Locate the zero mark on the vernier scale (it's the red zero on the bottom scale above), look immediately up at the main scale and start moving left until you see the first mark on the main scale "at or to the left of" the zero mark on the vernier scale.
1. In the example above, the vernier zero mark is at or immediately after the 0.0cm mark on the main scale. We now know our measurement is somewhere between 0.0cm and 0.1cm.
Find the line on the vernier scale that aligns perfectly with any line on the main scale. Record this vernier scale reading.
1. Notice that the mark on the Vernier Scale at 0.0 aligns perfectly with a line on the main scale above, so we will capture 00 as our extra precision.
Add the two readings together to get the final measurement.
1. 0.0 plus 0.0 = 0.000cm.

The vernier scale is used for precise measurements.

Check out this Wikipedia entry.

The larger/fixed scale is called the "main scale" or "linear scale".
The smaller/sliding scale is called the "Vernier scale".
Many Vernier calipers show both imperial (e.g. inches) and metric (e.g. centimeters) units. In the picture above, the top portion of the scale is imperial and the bottom is metric. You will only ever be reading one side of the scale (e.g., top or bottom). In our example, ignore the top/imperial section.
The interactive Vernier calipers on this page can measure things between 0.000cm/0.00mm and 15.000cm/150.00mm long out to precision of 0.002cm/0.02mm. This precision is approximately the diameter of the average human hair!
We know it's 0.02mm because:
1. The main scale has markings for every 1mm.
2. The Vernier Scale has 50 units on it (10 numbered units with 5 un-numbered units between each one).
3. So the Vernier Scale divides each millimeter on the main scale into 50 sub-units, 1/50 = 0.02.