How did Pythagoreans connect the tetraktys to a triangular shape?

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Marble bust of Pythagoras, the ancient Greek mathematician and philosopher, showing a bearded man with a head covering and calm expression. — Pythagoreanism Greek philosopher and mathematician Pythagoras, portrait bust.

In Pythagorean philosophy, the tetraktys is a sacred symbol representing the decad, which is the sum of the first four integers (1 + 2 + 3 + 4 = 10).1

Pythagoreans connected this concept to a triangular shape by visualizing the numbers as a "perfect triangle." In this geometric arrangement, a single point is placed at the apex, followed by two points in the second row, three in the third, and four in the base, totaling ten points. This configuration reflects the Pythagorean belief that numerical proportions and geometric forms are the fundamental reality underlying the order of the cosmos.1