Decad as vesica piscis
Here we see another form of the Pythagorean decad, highlighting its hexagonal nature.
Geometries such as this have been used by people throughout history—Pythagoreans, Freemasons, &c.—to justify decimalism. This is a false understanding of the decad. In both this diagram, as well as in the Tetractys, it is clear that the fundamental organizational principle is hexagonal, not decadian. The concept of ten emanates from the underlying reality of the hexagon—not the other way around.

Let ABCDEF be the given circle. It is required to inscribe an equilateral and equiangular hexagon in the circle ABCDEF.
The following is a brief survey of some elemental properties of hexagons, and why they might be useful. It is not intended to be a comprehensive treatment of the subject. My specific concern here is with the mathematical properties of hexagons, and, to an extent, their role in the natural world. I have avoided discussing hexagons as they pertain to human culture, religion, history, and other "local" concerns, though there are many fascinating instances of hexagonality and sixness in these areas, and they will no doubt be treated more fully elsewhere at another time.