Theory:

Let us take a moment to look at our surroundings. The things we see have different shape and textures.
 
The books we use have sharp lines, wherein the bowls we use have curves.
 
Let us look at the representation given below for a better understanding.
 
All the things we see have different measures of lines and curves.
 
But is it possible to measure different lengths of lines?
 
Yes, we can measure a line in different ways. Let us look at each of them in detail.
Measuring a line
We can measure a line in three ways, and they are:
 
1. Comparison by observation
 
2. Comparison by tracing
 
3. Comparison using Ruler and a Divider
 
Comparison by observation:
 
In a few cases, we can spot the longer and shorter lines easily by sight.
 
But the verdict may not be accurate in many cases in many cases.
 
Let us look at the representation given below for a better understanding.
 
1.svg
 
We can spot the shorter and longer lines in figures \(1\) and \(2\) easily in the image given above.
 
But, in figure \(3\), we would need a detailed examination in finding the longer and shorter lines.
 
In other words, lines with closer length differences cannot be accurately differentiated by sight and need a closer examination.
 
Comparison by tracing:
 
We can also identify shorter and longer lines by tracing the lines using tracing paper and placing them on each other.
 
2.svg
 
If the traces of \(\overline {PQ}\) and \(\overline {RS}\) are placed on each other, then we find that \overline {RS} is longer than \(\overline {PQ}\).
 
Comparison using Ruler and a Divider:
 
We measure lines using a Ruler and a Divider most commonly.
 
All our geometry boxes contain rulers and dividers.
 
3.svg
 
A ruler is divided into \(cm\) and \(mm\). The longer lines denote the centimetres on the rulers.
 
Each of the centimetres is split up into \(10\) parts called millimetres.
 
That is, \(1\) \(cm\) \(=\) \(10\) \(mm\).
 
The readings on the scale help us measure the length of the line.
 
We place the zero mark on the ruler on one end of the line and read the measurement against the other end.
 
Let us look at the representation given below for a better understanding.
 
4.svg
 
The zero mark on the ruler is placed at \(P\), and the mark at \(Q\) denotes the measurement of \(\overline {PQ}\).
 
Here, \(\overline {PQ}\) is \(5\) \(cm\) in length.
 
However, there may be minute errors while measuring lines rulers due to the thickness of the lines on the rulers.
 
In such cases, we can use dividers.
 
Let us look at the representation given below for a better understanding.
 
5.svg
 
Here, both the ends of the divider are placed on \(A\) and \(B\).
 
Then, with the exact measurement intact, one end of the divider is placed on the zero mark of the ruler, and the marking next to the other end of the ruler gives the measurement of the line segment \(\overline {AB}\).