A step by step instructable on creating perpendicular lines
Create a line using a straight edge, plot 1 point on each side of the line.Label one point A and one point B
Start with the point of a compass on point B, and make an arch.
Keep the compass the same, and copy the arch onto the other side of line AB
Still keeping the compass the same, copy the arch but start with the pint of the compass on point A. Make sure that the 2 arches intersect
Copy step 4 onto the other side of line AB
Draw a line using a straightedge, that crosses through the intersections that the arch's made to create two perpendicular lines.