complementary expressions

From Ott's Fundamentals of Catholic Doctrine, p. 64:

The co-ordinating formula (filioque) and the subordinating formula (per filium) concur essentially, in so far as they both attest that both the Father and the Son are the Principle of the Holy Ghost and they also complement each other. While in the former the unicity and the indivisibility of the Principle are above all expressed, the latter effectively stresses that the Father is the Primitive Principle (cf. Augustine, De Trin. XV 17, 29: de quo procedit principlaliter), and that the Son as "God from God" is the Derived Principle, in so far as He, with His Essence, also receives the power of spiration from the Father. Cf. D 691.

Note to self: learn about the Filioque controversy.