The certain digits are determined by noting the last graduation mark that the sample reaches.
In the example shown, the orange solid reaches the fifth graduation mark past the 2 cm mark. Since each graduation mark is 0.1 cm, the fifth mark past the 2 cm mark is the 2.5 cm graduation. And, since the solid does not reach the 2.6 cm graduation, the certain digits are 2.5 cm. Once the certain digits are determined, then the uncertain digit MUST be determined.
