Chords with different bass and non-diatonic chords inside a progression

Question

I'm studying music by myself and I'm dealing with several doubts: one of these is, are there some criteria for using a chord with a different bass in a chord progression? And how should I use it? Also, are there some specific ways to use non-diatonic chords inside a progression? I was analyzing yesterday this song by Bruno Mars, which I really like (D Major key), where you can find for example both Gmaj7 and Gm chords, and the second one of course is not constructed in the key of D major. I just want to understand a bit better the whole thing!

You cand find the sheet music here:

Bruno Mars - Versace on the floor

https://musescore.com/user/16069731/scores/3461536

Thank you in advance!

Answer 1

You're not going to understand music by working outward from 'the rules' I'm afraid! But you're doing the right thing, you're observing some real-world music that goes beyond simple diatonicism. (And, believe me, even popular music - let alone the heavy stuff at Symphony Hall - goes WAY beyond it all the time!)

The key, the home scale of a tune and the chords that can be made from those notes, are a framework. Not a restriction. You've discovered that both IV (G chord in the key of D) and iv (G minor chord in the key of D) sound good. So do a whole lot of other chords. In fact, it would be trivial to demonstrate how just about ANY chord could fit well into a sequence in D major!

I'll just offer one 'rule'. Don't think of chords as entities. 'In key x, we can use chords x,y,z...' Look at the NOTES in a chord. We're in D major. We know G major is a basic diatonic possibility. Perhaps the next chord is A major. Now, rather than taking the B in G major directly to the A in A major, what more natural than to let it slip down half-way first, to Bb! That's why the mini-progression D, G, Gm, A sounds so natural.

Answer 2

Trying to answer the first part of your question. I imagine you mean 'slash chords'. Chords which have two letter names. The first is the chord name itself, the second the lowest note played. This can be one of the chord notes itself, or another note altogether.

Let's take the former. Sometimes it's not the root position that's needed. With C major - C E G - if a 1st inversion is needed, it's shown by C/E.

The latter is a bit more complex, as the note after the slash doesn't necessarily belong to the chord. Like in this sequence: C, C/B, C/Bb, F/A. It could be said that the 2nd chord is Cmaj7, 3rd C7. But what's important is the bass line, which is chromatic. Those chords could be played just as 4/3 note chords, but the chromatic bass line would possibly be lost doing that.

The other way is something like C/D, a sort of 'CaddD', except here, the D is under not above the chord.

Moving on to the other part: Laurence has it pretty well covered, I'm just adding that it's been found (therefore belongs to theory) that another whole set of non-diatonic chords work well. It's from the parallel key. In your case, D is the key, so Dm is the parallel. That explains to a degree where the Gm pops up from.