Green Guides

Materials:
Pen or pencil if you want to erase extra marks
Object with straight edge
Flat see-through plastic object
2 pins or thin pointed nails
Cardboard

Find a flat see- through plastic object and cut out a strip from it.
*Make sure it is plastic that does not stretch.

Draw a straight line with a straight edge

Place the paper you want to draw on, on top of a piece of cardboard. In this guide a shoe box cover was used.
*If you are drawing on thick cardboard it is not necessary.

Place the plastic strip over the line
Push in one pin at the left end of the line
Push in the second pin on the line a little more than half way to the right

Remove the pin on the right

Put the point of you pencil or pen into the hole of the pin you removed and rotate the strip to draw an arc

Remove the strip

Put a pin in one of the holes in the strip and then push it through the right end of the line

Put the point of the pencil or pen into the second hole and rotate the strip to draw an arc

Use the straight edge to draw a line connecting the 2 places where the arcs meet

Result
You now have exactly perpendicular lines!

If you have drawn in pencil you can erase the arcs.