It’s hard to figure out what key a piece of music is in. There are a lot of conflicting answers from different music theory texts. To make matters worse, it’s not at all unusual for a song to change keys, even within a section or phrase. Even rock songs written by totally naive songwriters can be full of key changes. So a lot of the time, you aren’t trying to figure out the key for the entire song; you’re figuring out keys for particular passages.
The good news is that while figuring out keys is complex, it’s not impossible. Before you can do it, you need to know what all the possibilities are, and you need some tools to help you in your analysis. I’m assuming here that you don’t have sheet music of the tune you’re trying to figure out, but you do have an audio recording. You’ll want a program that can loop and slow down different sections. I recommend Transcribe for this purpose. Audio editing tools like Ableton Live and Pro Tools work too.
Figuring out the root
The root is “home base” for the key. The best way to figure it out is trial and error. There are only twelve possibilities; put the passage on a loop and try them all. Often the root is the note that the melody starts or ends on, or repeats frequently. The root is also the likeliest note for the bass to be playing. Neither of these are hard and fast rules, though; you need to let your ear be the final judge. A teacher can help you develop your instinct through ear training.
Simple major and minor
In traditional western classical and folk music, figuring out keys is relatively straightforward. There are twelve major keys and twelve minor keys. If the piece or section feels happy, it’s probably major, and if it feels sad, it’s probably minor. Once you’ve figured out the root, you’re in business. The key signatures in western notation denote either major scales or their relative minor counterparts.
Jazz theory gives another handy method for identifying keys. In jazz, keys tend to change a lot, and it’s not uncommon to pass through a key without ever landing on the root. Many jazz standards have sequences of ii-V progressions that don’t resolve. For example:
| C-7 F7 | Bb-7 Eb7 | Ab-7 Db7 | G-7 C7 |
How do you make sense of something so complex? Jazz says: look at the dominant seventh chords. Whenever you see one, you’re in the key one slot counterclockwise on the circle of fifths.
So in the example, F7 indicates that the first bar is in Bb major — you can play the Bb major scale over C-7 and F7 and it will sound good. The Eb7 indicates that the second bar is in Ab major. The Db7 puts the third bar in Gb major, and the C7 puts the fourth bar in F major.
Dominant chords can indicate minor keys if they have “minor-sounding” extensions in them like b9, #9 and b13. Half-diminished ii chords suggest minor keys too. So if you see this:
| C-7b5 F7b9 | Bb-7b5 Eb7#9 | Ab-7b5 Db7b9 | G-7b5 C7#9 |
then you’re looking at the keys of Bb minor, Ab minor, Gb minor and F minor, respectively. See a good jazz theory text for more details; I recommend The Jazz Theory Book by Mark Levine.
Blues music makes the key situation more complex, because it combines features of both major and minor. Blues mixes the major and minor third, the natural and flat seventh, and characteristically major-key and minor-key chords. Also, blues uses dominant seventh chords in non-functional ways, so you can’t really use the jazz analysis on them. I think it’s best to think of “blues keys” that are distinct from major or minor. So the key of “C blues” will use chords like this:
C7 Eb F7 F#dim7 G7 Bb
Melodies in C blues will combine C major and the C blues scale. There’s also minor blues, but that follows traditional classical minor-key rules more closely.
Modes and other exotica
Major, minor and blues will cover you for most of the material you’ll encounter in western pop, but there are still some tunes and passages that will continue to defy analysis. Music from non-western cultures uses plenty of scales not mentioned above, and western practice has absorbed a lot of them, especially the ones that happen to map onto modes of the major scale. Western music also occasionally ventures into modes of the harmonic and melodic minor scales.
There are also the so-called synthetic scales: whole-tone and diminished. It’s vanishingly unusual to hear them in pop songs, but they do show up in jazz, classical and film score music. Hollywood leans heavily on a scale I learned as “harmonic major” — like harmonic minor but with a major third:
C D E F G Ab B
This scale is rarely discussed in music theory texts, but it’s handy if you want to create a feeling of epic mysticism, like in the Lord Of The Rings movies.
Putting it all together
This flowchart shows all of the scales you’re likely to encounter in contemporary American music, at least the more mainstream and accessible stuff. Find yourself a piece of music whose root is C. Try out different notes on top of it and follow the arrows until you hit your scale.
For example, let’s take “I Can’t Go For That (No Can Do)” by Hall and Oates.
Take a listen to the verses. The root is C. There’s G in there, whereas Gb sounds peculiar. The section is minor, so Eb fits and E doesn’t. Continuing out, Bb fits better than B, and A fits better than Ab. So the key here is C dorian. The prechorus — “I’d do anything you want me to do” — is more complicated. It has C and G, and the third is E, rather than Eb. But the different chords in that section use both B and Bb. The prechorus switches back and forth between C major and C mixolydian. See what I mean about key changes being common?
If you’re trying to figure out something and the root isn’t C, my chart won’t be much help unless you transpose everything. Someone want to commission me to do this chart in all twelve keys? Get in touch.
Music theory excites nerds like me, but it makes a lot of musicians miserable. It’s best to learn it in the context of actual tunes that you like. Each scale has its own unique emotional color; puzzling them all out and comparing their qualities can be great inspiration for writing and improvising your own stuff. Happy transcribing!