Practical music theory: what is a key?

May 22, 2018
Practical music theory: what is a key?

When it comes to the practical elements of music theory that you’re likely to use every day, knowing the notes and chords that belong to a given key is one of the most useful.

You can certainly memorize a song without knowing the key, but there’s lots of situations where you’ll need that information. If you’re a singer trying to find a song’s sweet spot in your voice, sometimes you’ll need to find a new key and the corresponding chords. Virtually everyone will need to tell other players the key they’re in at some point. Maybe you’re trying to come up with a capo part and need to figure out what position and fingerings to use. If you’re learning to improvise, it’s really helpful to know which notes belong and which notes don’t. And songwriters certainly benefit from knowing which chords will work together well.

If you ever took piano lessons or a theory class, you learned that a key is defined by which seven notes we use (out of a possible twelve) to create a major or minor scale. The key of C major uses the notes C, D, E, F, G, A, and B: all the white notes on the keyboard, and none of the black notes. Played in sequence, that gives us a C major scale. The key of G flat, on the other hand, uses the notes Gb, Ab, Bb, Cb, Db, Eb, and F, and so is played mostly on the black notes. Every major key uses a slightly different combination of white and black notes.


We could look at the formula for creating a major scale, but that’s just doing the math.

It’s equally important to understand what “key” really means musically. It’s not just a mathematical relationship but also a sonic phenomenon, an expression of the almost gravitational pull notes can exert on each other. It’s easy to illustrate this. Listen to the following chord progression:

C – G

Are we done? It doesn’t sound that way! Notice how the G chord sounds unresolved. If we add a C chord at the end, you’ll hear the musical thought completed:

C – G – C

The chord progression is 1 – 5 – 1, a chord built on note 1 of the scale moving to a contrasting chord built on note 5. This creates the defining sound of Western music: leaving a stable musical home for an unstable tension chord that then resolves, taking us back home. This sense of harmonic tension and release creates movement when we string chords together. And that resolution tells the ear what key you’re in.


This is an important insight. It’s easy to think of “key” as “notes of the scale”: if a note belongs to the scale, it’s in the key, and only notes from the scale belong to the key. This is technically and mathematically true, but musically misleading. It’s more helpful to think of a “key center” or tonal center, the note or chord the music resolves to. The notes of the major scale are primary, but that doesn’t mean we can’t use the other notes too. A song can use notes from outside the major scale without changing the sense of musical “home”.


For example, listen to the following two chord progressions:

C – Am – Dm – G

C – A7 – D7 – G

Both should be familiar sounds, but suggest different types of songs. The first wouldn’t be out of place in a 1950’s doo-wop ballad, while the second has a little more of a jazz or blues vibe. Both are firmly in the key of C, but the second progression contains two notes that are outside the C major scale. The chords work because the different notes actually point the ear more strongly to the next chord. In fact, there are two different but related “pulls” at work.

The A7 chord contains a C# note, which doesn’t belong to the C major scale but creates a smooth melodic transition between C and D. Since we’re headed to a D chord, the harmony does the same thing. In fact, A7 is the 5 chord in the key of D and so sets up the D7 the same way G sets up C: a 5 – 1 or “dominant-tonic” resolution. Conveniently enough, D7 is the 5 chord in the key of G! So this simple chord progression contains several nested gravitational connection: C walking to C# then resolving into D as part of the 5 chord, followed by F# walking to G in exactly the same way before the whole business concludes as B walks to C.

All this theory can seem very complicated at first.

The simple idea you should be focused on right now is that chords can point to other chords, and that the sense of pull comes from melodic motion AND from the amount of tension or release each chord creates.

Let’s take a look at another variation with even more tension:

C – Eb dim7 – Fm – G

The second sequence includes even more notes that don’t belong to the C scale, but we still hear the same sense of resolution back to C major. So the tonal center is still C, and the key is still C major. But while progression 1 is “diatonic” – using only notes from the scale – progression 2 is “chromatic”, incorporating notes from outside the scale. If you’re not sure if you hear this, take a step back and listen for “simple” or “busy” sounds.



The use of chromatic notes gives progression 2 a different sound and atmosphere – the “busy” sound – but the basic flow is very similar. We might say that the two progressions have the same structure but different coloration. Let’s compare them using the Nashville Number System, a form of notation that expresses chords through their mathematical (and sonic) relationship to each other:


1          6m       2m          5


1        b3dim       4m         5

The notes of A minor, the “6 minor” of the key, are A, C, and E. The notes of Eb diminished, which replaces it in progression 2, are A, C, Eb, and Gb. That means we’ve kept two of the three notes of the chord, and the new notes are only a scale step away from the original. E drops a half-step to Eb, and Gb could also be thought of as F#, a whole step higher. If this is confusing, just think of the letters and it’s not hard to see that the notes are close together: E and Eb, E and F#.


This is the big idea, and it was a revelation to me when I understood it: the ear will accept any note or chord if it can find a way to make a connection to the sound before it and after it.

In other words, the relationship between the chords and the notes of the scale is important, but the relationship between the individual notes that form each chord is what makes the music hold together. We can use notes from outside the key (scale) as long as the ear can find a way to connect them. Even within the chords, the notes form a melody.

Sometimes that forms the main melody of the song. Listen to the opening and verse of “Something” by the Beatles. The opening guitar lick brings out the chromatic notes of the opening chord progression, and the vocal melody does the same thing in the verse! The flow of the song is seamless, and we never hear a note that sounds “wrong”.

When you start to understand this concept more thoroughly, it really enhances your your overall understanding of music. Learn the mathematical relationships and memorize the “key signatures” – the notes that belong to each key. But also start listening for simple and busy sounds, tension and release, and melodic connection between chords. You’ll find that it opens up a world of musical possibility.

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