
Start with a line and point K on that line. 

1 
Set the compasses' width to a medium setting. The actual width does not matter. 

2 
Without changing the compasses' width, mark a short arc on the line at each side of the point K, forming the points P,Q.
These two points are thus the same distance from K. 

3 
Increase the compasses to almost double the width (again the exact setting is not important). 

4 
From P, mark off a short arc above K 

5 
Without changing the compasses' width repeat from the point Q so that the the two arcs cross each other, creating the point R 

6 
Using the straight edge, draw a line from K to where the arcs cross. 

7 
Done. The line just drawn is a perpendicular to the line at K 
