In astrology, Pisces rules the Twelfth House of the Unknown; what shan’t be spoken and thus remains unknown. In other words, it rules the system beneath the concept.
Pisces is linked to institutions. An institution is defined as an established law, practice, or custom. Including: government, marriage, banks and universities. Institutions, similar to mountains, are ancient yet clearly defined.
A planet in Pisces actively works towards something greater than itself. A planet in the Twelfth House has roots deeper than a tree. And having a planet in Twelfth House Pisces is wonderfully basic.