In this case, the prototile is an elongated irregular enneagon, or nine-sided figure. These 6 symmetries can be seen in 6 distinct symmetries on the enneagon. The "regular enneagon" has Dih symmetry, order 18. The regular enneagon can tessellate the euclidean tiling with gaps. A polygon with nine sides is called a nonagon or enneagon. In geometry, a nonagon () or enneagon () is a nine-sided polygon or 9-gon.