Most of us learn to draw a circle with a compass at an early age, and can manage it without too much difficulty using a piece of string and a pencil. Perhaps you already know how to draw an ellipse using string as well (see Tangente no. 221). A parabola can be drawn as well, but you will need to add a ruler and a set square to your toolkit. Choose a piece of string the same length as one side of your set square (the length AH in the diagram). Tie it to the focus F of the parabola and to the endpoint A of your set square. Place the set square against the edge of a ruler, which will act as the directrix. Then slide the set square along the ruler, keeping the string taut with a pencil. You will see the focal-directrix definition of a parabola come to life, since MH = MF. If your string is not the same length as the side of the set square, a similar construction is still possible, but you will need to work out how to position the ruler parallel to the directrix.
A little origami
---------------------
If you are not entirely comfortable with a pencil and string, you can always try a construction that requires drawing nothing more than a single point. Take a rectangular sheet of paper, A4 for example. Mark a point F on the sheet, then choose a point M on one of its edges.
