I’ve recently had one of those ridiculously pleasing light bulb moments…
I’ve finally found out why, in music, the subdominant is so called. Having begun playing guitar in about 1970 I have always been aware of the tonic-subdominant-dominant ‘three chord trick’ and have probably known them by those names for ever too.
I have always assumed that the subdominant was so called because it is one tone beneath the dominant. But no, it isn’t, and those of you who have always known why it is ‘sub-‘ will be laughing at me for my dimness.
The dominant itself is a 5th above the tonic. (In The key of C, where C is the tonic, G is the dominant.) The subdominant is a 4th above the tonic. (F, in the key of C.)
But moving down from C to the F below the interval is a 5th. In other words, the sub-dominant. A 5th down rather than a 5th up.
Boy, am I glad that’s cleared up, after forty five years of accruing quite a lot of music theory. I have to thank Philip Ball and his wonderfully clear exposition of a lot of the science and theory of music: The a Music Instinct: How Music Works And Why We a Can’t Do Without It.