Why did Pythagoreans associate even numbers with the female principle?

The Pythagoreans associated even numbers with the female principle as part of a broader dualistic doctrine of opposites. In their metaphysical system, they categorized the universe into pairs of opposing concepts, such as limit and unlimited, rest and motion, and male and female.1

Within this framework, the odd numbers were identified with the "limit"—an active, ordering force—and were associated with the male principle. Conversely, even numbers were associated with the "unlimited," representing a more passive or receptive quality, which the Pythagoreans linked to the female principle. This classification was part of a larger "table of ten opposites" that sought to explain how diverse elements of the cosmos "fit together" to create harmony and order.12