Video narration
How can a vernier caliper measure a length between ruler marks? The jaws hold the object while a sliding vernier scale moves beside the main scale. Read the last main-scale mark before the vernier zero, then find the vernier line that aligns. The vernier is not a second reading: its slightly different spacing makes alignment reveal the extra fraction. That's how: the main scale gives the whole measurement; the aligned division times the instrument's least count adds the fraction.