Ten points arranged in four rows of one, two, three and four, summing to the decade. The tetractys is the Pythagorean figure of the number ten and the compact statement of the whole doctrine: the one that is not a number, the two of the first division, the three of the first complete figure, and the four of the first solid. The Pythagoreans swore by it, and the oath calls it the source that holds the root of ever-flowing nature.
Sacred Ratio
10 (the fourth triangular number)
Construction
Place one point, then two beneath it, then three, then four, each row centred on the one above. The rows are the first four integers, their sum is ten, and ten is the number of the decade, so the figure is both an arrangement and an argument. It can also be read as the first four triangular numbers as a single triangle, and as the 1:2:3:4 ratios that give the octave, the fifth and the fourth.