From time to time, I’ll get a distressed email from a stranger on the Internet asking some sort of music theory question. I’m always happy to oblige and answer these inquiries, often with a more long-winded explanation than I think they’re expecting. This is my way of giving back to the community! (I’ve come to terms with the fact that I’ll probably never be asked to speak on a podcast or testify in court as an expert witness.)
Here’s one from the other day:
I’m ashamed I need to ask this question.
I need a compositional justification for the existence of the iv chord.
Ex: Fm in key of C.
I have played it and composed with it for decades,
but I don’t really understand it.
I have a some grasp of other “altered” diatonic chords
ex: II major (secondary dominant function)
ex: A7 in key of C
Just don’t understand the iv.
Can you help me please
Thanks so much
And my response, in case anyone else is wondering the same:
First of all, there’s no need to be ashamed! There’s never a straight answer to this sort of question. (And, perhaps unfortunately, you’re not going to get one out of me!)
There are several ways to conceptualize the derivation of a minor subdominant in a major key. The simplest of these, I would say, is that it is a simply an alteration of the diatonic (major) IV done for coloristic reasons to heighten the dramatic intensity of a particular moment. In this sense, the minor subdominant is a lot like a Picardy third (the raising of the third in a minor tonic triad, often appearing at the end of a piece that is otherwise entirely in a minor key). It makes the chord stand out but not so much as to sound out of place. It has a somewhat unexpected quality, but it usually retains its pre-dominant function and leads smoothly to some sort of V chord.
You can also think of it linearly in terms of the direction each chord member tends to move. The diatonic subdominant consists of scale degrees 4, 6, and 1 (FA, LA, and DO). The minor subdominant differs only at the third: b6 instead of 6 (LE instead of LA). In Common-Practice voiceleading, the sixth scale degree tends to resolve downward to the fifth. This is particularly true of the flatted sixth scale degree given its placement a semitone away. (You can think of the half-step resolution from b6 to 5 as being analogous to the resolution of the leading tone up to the tonic.) In this sense, the alteration of the subdominant to include b6 may be understood as a chromatic intensification of the voiceleading as the pre-dominant harmony moves to the dominant. In fact you will see this from time to time as a composer such as Brahms will first use the unaltered 6 and follow it immediately with a b6.
But here’s what I think (and feel free to take this or leave it): The minor subdominant doesn’t really need justificaiton. Remember that music theory, though often presented as a set of rules or laws governing how music works (or should work), it is actually backward-looking attempt to make sense of apparent patterns in music of the past. I’ll say that Brahms uses the minor subdominant to intensify the musical drama as pre-dominant moves to dominant, but the truth is that I have no real way of knowing if that was his intention. In fact, it seems more likely to me that he just liked the way it sounded! You yourself mentioned that you “have played it and composed with it for decades” and that, I think, is justification enough!
I hope this helps!