In geometry, a nonagon () or enneagon () is a nine-sided polygon or 9-gon. A polygon with nine sides is called a nonagon or enneagon. The triangle, quadrilateral and nonagon are exceptions. The U.S. Steel Tower is an irregular nonagon. The full Project Nonagon album was released in 2010. Examples include the regular pentagon, heptagon, and nonagon and the equilateral triangular bipyramid. The following is an "approximate" construction of a nonagon using a straightedge and compass. The nonagon can be constructed with 36 meccano equal bars. Upon its release, "Nonagon Infinity" received acclaim from music critics. The first NONAGON store opened on September and sold out within 3 days. Project Nonagon is the fourth studio album by the Seattle-based digital hardcore band Rabbit Junk.