So, what would you say is the key of this popular Bruno Mars/Mark Ronson tune?

First off, let's see if you can figure out the correct chords in the tune. Here's a hint - there are only 2 chords in the song. Give it a listen and try to figure it out before you continue reading.

Ok, have you checked out the tune? This style of music (retro funk with horn sections) is making a little bit of a resurgence lately, and really owes a lot of its origin to artists like James Brown and Rufus, among others (check out James Brown's song "Livin' In America" to hear what I mean).

Now back to the chords. Again, there are only two and they are Dmin and G7. That's to, that's the entire harmonic structure - back and forth between Dmin and G7. Now it's important to notice that Dm to G7 is the first part of a ii-V-I progression in the key of C. So, this song is in C major, right? Well, sort of, but not exactly.

You see, the song never resolves to C major. It just goes back and forth, over and over again, from Dm to G7. This is why most people will listen to the song and say that Dm is the primary key for this song. But the key of D minor has a key signature which contains one flat (Bb) because D minor is the relative minor of F major. And surely, there is no Bb in this song (in fact, B natural is played often in the horn parts). So, what's up? Is this song in the key of C major or D minor, or something else?

The answer is that both answers are somewhat correct. The key signature for the song would definitely be no sharps or flats - so key of C major, right? Not exactly. Since the song simply cycles back and forth without ever resolving to C major, this is not really accurate. The closer answer is that the song is in D minor, but not D natural minor - D DORIAN is the key of the song. And D dorian has a direct relationship to C major (it's built on the 2nd degree of a C major scale).

If I were telling someone the key for this song, I would probably say "Hey man, this tune is in D minor." But that's not a complete answer for the reasons stated above. This is one of those situations where the song is really based on modal theory. The two chords of the song are constructed from C major, but the chord scales for each chord would be D dorian (for Dmin) and G mixolydian (for G7).

How would you know this as a listener trying to pick out the key of the tune? Well, first of all LISTEN to hear the root movement of the harmony. Can you hear the bass player playing the roots of the chords? Where does it sound settled or resolved? 

Secondly, knowing the theory (ie, key signatures, relative major/minor, and modes) can help you make an informed decision as opposed to a random guess.

So there it is - C major definitely plays a role in the key signature of this song, but the song is based in the dorian mode of D minor.