The Partzufim: male and female, and the differences between them.
Partzufim differentiate into male and female — and the operational locus of differentiation is the Yesods.
Op. 72 specifies where in the Partzuf-anatomy male/female differentiation operates. Coupling becomes the operative mechanism for combined-flow.
The Yesod (the ninth Sefirah, the Foundation) is the structural locus of generation in any Partzuf. Male Partzufim have male Yesods (active, projecting); female Partzufim have female Yesods (receptive, gathering). When a male Partzuf and a female Partzuf couple, the Yesods meet; the male Yesod descends into the female Yesod; the cosmic effect is generated.
This specifies the operational mechanism of the Coupling-Pregnancy-Birth-Suckling cycle (Op. 66). The Coupling happens at the Yesods; the Pregnancy follows in the female interior; the Birth is the offspring's emergence; the Suckling is the post-birth growth. The Yesods are the structural-anatomical foundation of the entire downstream coupling-doctrine.
Op. 75 will say Atik Yomin's M and F are literally one body — the limit case in which the male and female Yesods are not differentiated* at the operational surface. Op. 110–114 (Abba and Imma) and Op. 117 (Zeir Anpin (Z"A)'s coupling-modes) operationalise the Yesod-differentiation across the named-Partzuf walk.