The diagram below shows the measurement of the length of a blue
bar using a ruler with 1 cm as its finest division.

Using this ruler, the length the blue bar is recorded to be 2.3
cm. In Section
1.6, you will learn that this measurement has 2
significant figures.

Let's analyze this measurement:

Upon inspection of the ruler, we observe that the finest
division is a 1 cm division.

We can always make an estimate of a reading to within the finest division.
Therefore, to the best of my judgement, I conclude that this particular ruler
is able to provide a reading accuracy to +/- 0.2 cm.
(Note: A plus or minus 0.2 cm reading accuracy means that a length measurement
using this ruler will contain uncertainty in the first decimal place.)

The end of the blue bar is between the 2 cm
and the 3 cm mark. We can estimate that the blue bar exceeds 2 cm mark by
approximately 0.3 cm.

The length measurement is determined to be 2.3
cm.
The digit '2' is a certain figure. The digit
'3' is an uncertain figure, or an estimate in
the measurement. Therefore, this length measurement has 2
significant figures. (Note: This measurement is accurate to 2.3
+/- 0.2 cm (i.e. 2.3 plus or minus 0.2 cm). It means that using this
ruler, the length of the blue bar is between 2.1 cm
to 2.5 cm.)