The unconstructible regular Polygon with nine sides and Schläfli Symbol . It is sometimes called an Enneagon.
Although the regular nonagon is not a Constructible Polygon, Dixon (1991) gives several close approximations. While the Angle subtended by a side is , Dixon gives constructions containing angles of and .
Madachy (1979) illustrates how to construct a nonagon by folding and knotting a strip of paper.
See also Nonagram, Trigonometry Values Pi/9
References
Dixon, R. Mathographics. New York: Dover, pp. 40-44, 1991.
Madachy, J. S. Madachy's Mathematical Recreations. New York: Dover, pp. 60-61, 1979.