Draw the shape of an ellipse of any shape on the board using a thin string stretched over two small nails driven into the substrate. In this case, hook the pencil to the edge of the taut string. The shape of the ellipse must be selected experimentally. It depends only on the length of the string itself and the spacing of both nails, on which it is unfastened. The string is longer, the wider ellipse. At the same length, larger spacing of nails causes, that the ellipse becomes more elongated (slim).
