Circles of nine are unfamiliar. But the octagon's division is highly familiar and quickly recognized. Septagons are not. Powerfully familiar and instantly recognized are the remaining hexagon, pentagon, square, and triangle. There is no twogon.
Triangle is the minimum polygon.
Triangle is the minimum-limit case. The triangle, being odd in number, cannot be bundled and thus remains not only the minimum polygon but the only inflexible, nonfoldable polygon. In Euclidean geometry triangles and other polygons were misinformedly thought of as occurring in two-dimensional planes. Only the triangle has an inherent and integral structural integrity. Triangles at their simplest consist experientially of one minimum-altitude tetrahedron.