In traditional music theory, your harmony is considered diatonic if it uses only the notes of the key it’s in, and there are plenty of great songs based around entirely diatonic progressions. However, once your song sets up an expectation of diatonic harmony, it gives you the opportunity to provoke an emotional reaction from the listener at any moment by introducing a chromatic chord – in other words, any chord containing one or more notes from outside the established key. The difficulty with using chromatic chords within traditional diatonic progressions, though, is that they can easily sound a bit awkward, as if you’ve just plonked down some random chord without any sense of trying to create a harmonic logic through the progression. And this particular Benson Boone song nicely demonstrates one of the most common solutions to that problem: a secondary dominant chord.
Here’s how it works. You choose any diatonic chord other than the key’s tonic chord, and then do a V-I perfect cadence to that chord as if it were the tonic chord (of a different key). So, in this song, for instance, the opening verse section from 0:07 to 0:33 (“For a while there it was tough… I think I might have it all”) is based around a simple repeated diatonic chord progression of IV-I-V-vi-IV-I-V. But the following section (“And I thank God every day… He can take away” at 0:34-0:47) introduces a secondary dominant chord of D major, creating a fleeting perfect cadence to that pattern’s Gm chord, the vi in the progression – giving us, in the process, a nicely evocative chromatic F# note that lends extra emotional weight to the words “the girl He sent my way”. Furthermore, the juiciness of this chord has been further intensified by embellishing it with the diatonic seventh and flattened-ninth notes of C and Eb, lending the chord a strong diminished-seventh flavour, given that the root D note only appears in the lead vocal.
Now if you’re wondering whether I should instead be calling this an F# diminished chord (with an anticipatory vocal D note) rather than a first-inversion D flattened-ninth chord, you’re not alone – it’s a topic I’ve seen classical harmony teachers get quite worked up about on both sides of the debate. As you can probably tell, I’m more in the ‘flattened-ninth’ camp, because it seems to make more sense musically in terms of the voice-leading (ie. where the individual notes in the chord progress to) – I’d expect the major third of a dominant major chord to rise, and its flattened ninth to fall, as they do in this instance, so irrespective of how much it looks like a diminished seventh, I think it functions more like a dominant chord. Let’s be honest, though – these are a pretty wafer-thin distinctions in practice, so I’m not about to unfriend anyone over it! As far as I’m concerned, the most important thing with any music theory is that it’s useful for your music-making, so if you get best use out of this kind of chord by thinking of it as a diminished seventh, then that’s definitely how you should think about it – whatever my thoughts on the subject!