The middle pillar of the Tree is that of Balance. It contains the tree's balancing aspects, the union of opposites[1] that harmonizes the actions of the other two pillars. In it are the three Sephiroth Kether כתר, Tiphareth תפארת, and Yesod יסוד. ...Oh, and that quintessential manifestation of seemingly unbalanced opposites, us: Malkuth (מלכות).

Of the Mother letters, this corresponds to Alef (א), which in the Gra tree also connects the Sephiroth Chesed and Geburah.

Sefer Yetzirah likens the three mother letters to a scale: a pan of merit, a pan of liability, and the tongue of decree that decides between them. The middle pillar is that balance — not active, not receptive, but instead both and neither.