The scholastic theologians, teachers that they were, came up with a numbered mnemonic for keeping the doctrines on the Trinity in mind:
1 nature: deity
2 processions: the Word and the Spirit. The Word proceeding from the Father, and the Spirit proceeding from them both. Thus we are led to...
3 persons: Father, Son and Spirit. And the two processions lead to relationships imagined from both directions, leading to...
4 relations: paternity, filiation, and active and passive spiration. And we add to these 4 relations the unbegottenness proper to the Father, we get
5 notions: paternity, filiation, active and passive spiration, and unbegottenness.
Thus we come to the famous frustrated formulation of theology students everywhere:
and ZERO understanding!