1. In order to draw parallel lines
HOW TO USE SET SQUARES
In order to draw parallel lines

2. Set squares are useful for drawing parallel lines and perpendicular lines.
Lines that lie in the same plane and never meet one another are said to be parallel lines.
In the next diagram,  line AB is parallel to line CD. (in fact, the correct nomenclature is line a, b, c, …always in lower cases.
                                                                    
In the next diagram, the line AB is parallel to the line CD. This is indicated by the similar arrows.
          

3. Right position of templates
There are a lot of possibilities to choose the appropriate position of set-squares. The easiest is to use the right angle of each template leaning on line (in this example, s or q) and to move it over the ruler.

4. This is very similar to...
A train that goes by railroad. If your train (the set-square) comes off the railroad track, you have to put the ruler back on the track and start again.