Line Segment
A line segment is bounded by two end-points. This makes it possible to measure its length with a ruler.
If we know the length of a line segment, it becomes possible to represent it by a diagram.

Construction of a line segment of a given length
Suppose we want to draw a line segment of length 4.7 cm. We can use our ruler and mark two points A and B which are 4.7 cm apart. Join A and B and get AB . While marking the points A and B, we should look straight down at the measuring device. Otherwise we will get an incorrect value.
Step 1     Draw a line l. Mark a point A on a line l.

Step 2     Place the compasses pointer on the zero mark of the ruler. Open it to place the pencil point upto the 4.7cm mark.

Step 3     Taking caution that the opening of the compasses has not changed, place the pointer on A and swing an arc to cut l at B.

Step 4   AB   is a line segment of required length.

Constructing a copy of a given line segment

Step 3     Draw any line l. Choose a point C on l. Without changing the compasses setting, place the pointer on C.