J.W.S. Rayleigh

On any note taken as a key-note, or tonic, a diatonic scale may be founded, whose derivation we now proceed to explain. If the key-note, whatever may be its absolute pitch, be called Do, the fifth above or dominant is Sol, and the fifth below or subdominant is Fa.

Rayleigh, John William Strutt, The Theory of Sound, Volume 1, Dover Publications, New York, p8 (first published in 1877)