Imagine an equalteral triangle composed of points. Like a pyramid the apex is a single point and there are four levels. The foundation is composed of four points. So altogether there are ten points. This is the tetractys; a mystical symbol to Pythagoras and initiates into the inner pythagorean mysteries swore an oath over it. The design was sometimes known as the Tetrad, or Tetractys of the Decad.
Like Dee’s monad glyph, the Tetractys symbolised various strata of concepts. The simplest was its practical mathematic impact. It must be remembered that Pythagoras taught that numbers were the divine language of the cosmos. Numbers were endowed with mystical insights. On a metaphysical level it was a seed that gave growth, and as the source of ever-flowing nature it was harmony and represented the principles of the natural world.
The design influenced cabalist thought, (especially the Tetragrammaton and possibly even the cabalistic tree of life). It is thought that the pythagoreans also used the tetractys to define a musical scale.