Drawing perpendicular to a line through a point not on it – Using ruler and set-square

Let’s take an example

1. Given a line l
   with point P not on it

2.Place set square on the line so that line is exactly above the set-square

3. Put ruler on the hypotenuse side of the set square
   
   
   
   
4. Keeping the ruler in the same position,
   slide the set-square till one end of the set-square passes through point P
   
5.  Draw a line along the edge of the set square, keeping set-square firm
   Let the line intersect line l at Q

Thus, PQ is the perpendicular to l