On instruments such as vernier gauges or micrometers there is a second magnified scale for quantifying values that lie between markings on the main scale. This is a similar in principle to adding digits to the digital display in the previous example and as before it can improve resolution but there will always be a limit to this approach and therefore there will always be some uncertainty in any measured values.
To read a value from the vernier guage you first read the value on the main scale at the point where the beginning of the smaller scale lies along the main scale.
you then read off the value on the smaller scale from the first marking that lines up with one of the markings on the main scale. You put 0.0 before this digit and then just add it to the value obtained from the main scale