How to Find the Key of Any Song from Its Chord Progression

Here's a brief overview of the cycle of fifths and how to use it to determine a song's key from its chord progression and which scales will work best to improvise over it. This doesn't cover every song, but it works in the vast majority of cases.

I thought this was going to be a quick topic, but it ended up being a jam-packed 11-page lesson!

How to find the key of any song from its chord progression

One of the most common questions I hear from guitarists is, "How do I know what scale to use to solo over this song?" At one level, this is a different way of asking, “What key is this song in?” It’s a good question…and I’m glad you asked!

In this lesson, I’ll share a method for mapping a song’s chords to its key. Like everything else in music, there are nuances and exceptions, but this method works flawlessly for many, if not most, songs. When it works, it also tells you a lot about the function of each chord in the progression, and that’s valuable information you can use to choose a scale (or scales) for your solo.

The cycle of fifths and relative minors

The cycle of fifths is one of the most useful tools in music theory for understanding how chords and keys are related. At its core, it’s a circular arrangement of the twelve notes in Western music, organized so that each note is a perfect fifth above the previous one as you move clockwise. The note names in the diagram are variously used to represent notes, triads, scales, and keys. I’ll also show you how to use them to think about modes.

The construction of the cycle is simple. If you start on C and move up a perfect fifth (seven semitones), you land on G. Move up another perfect fifth, and you reach D. Continuing this process gives us the twelve tones of the chromatic scale, looping back to C after passing through all twelve notes:

C → G → D → A → E → B → F# → C# → G# → D# → A# → F → C

Moving counterclockwise, each step is a perfect fourth (five semitones up) instead of a fifth, which is why the cycle of fifths is sometimes also called the cycle of fourths when read in reverse.

A key is a collection of seven notes drawn from a parent scale, and a major key contains the notes from the corresponding major scale. A key signature is just a shorthand way of notating which of the notes in a key are sharp or flat relative to the C major scale. Every major key has a relative minor that shares the same key signature (i.e., the same seven notes). The relative minor of a major key is always found a minor third (three semitones) below the major root. On the cycle of fifths diagram, the relative minor keys form the inner ring.

For example:

C major (C-D-E-F-G-A-B) shares all its notes with A minor (A-B-C-D-E-F-G).

G major (G-A-B-C-D-E-F#) shares its notes with E minor (E-F#-G-A-B-C-D).

This relationship is important because it means that a song in a minor key draws its chords from the same position on the cycle of fifths as its relative major key. As an aside, it’s also worth knowing that the scales for any two adjacent keys on the circle differ by only one note.

But don’t worry if you didn’t follow all of that. You don’t need to worry about most of it to use this in practice.

Here’s why this matters: If a song’s chords are entirely diatonic (meaning they all come from a single major scale or one of its modes), those chords will all fit within three adjacent positions on the cycle of fifths. Some can be major, and others can be minor, but they’ll all fit inside a wedge of three adjacent relative major/minor pairs. This ignores the rarely-used diminished chord built on the seventh scale degree.

If the chord progression is based on the major scale, we often refer to the chords using Roman numerals. When we do this, we use uppercase letters for major chords and lowercase letters for minor chords, and if we build triads based on the first six scale degrees of the major scale, we get I, ii, iii, IV, V, and vi. In C Major, that’s C, Dm, Em, F, G, and Am. In the cycle of fifths, these chords appear as neighbors in the cycle-of-fifths wedge.

The modes also fit into this framework. If you build one of the major or minor diatonic modes from the root of one of the chords in the cycle-of-fifths diagram, you can find five other modal neighbors nearby, always arranged in the same way.

The three major and three minor modes in a key all appear as neighbors in the cycle-of-fifths chart

Don’t worry if this doesn’t make perfect sense yet. There are concrete examples coming soon.

Step-by-step method for identifying the key

Here’s how to figure out the key of a song from its chord progression in four easy steps.

Step 1: List the chords in the song

As an example, let’s take “Perfect” by Ed Sheeran. It’s a classic four-chord song with the chords G, Em, C, and D. Ed mixes in a few variations such as Em7, Cadd9, and Dsus4, but we can safely ignore those particular extensions. (It’s beyond the scope of this lesson to exhaustively look at chord variations and extensions, but you can always treat a m7 chord as a minor chord for this analysis, and you can always ignore an add9 on a major triad. sus4 is slightly more nuanced, but for the most part, ignore that too.)

For our purposes, strip away everything except for the major or minor triad at the heart of each chord. The vast majority of the time, this will still give you the correct answer.

Step 2: Locate these chords on the Cycle of Fifths

Next, look to see where the chords of the song fall on the cycle of fifths. For this song, they fit nicely into three adjacent wedges. Sometimes, the chords will “spill over” into an adjacent wedge, or the picture will look more complicated than this, but most “four-chord” songs fit this pattern:

At this point, we can hypothesize that the song is in one of four keys (albeit all with the same notes!), based on the modal wedge:

The three major and three minor modes in a key all appear as neighbors in the cycle-of-fifths chart

So this song could theoretically be in C Lydian, G Ionian (major), E Aeolian (minor), or D Mixolydian.

Step 3: Identify the likely tonic (home) chord

To pin down the key, you can use a few rules of thumb:

Often, the I chord signifies the key, so your first guess should be the major key in the middle slot of the wedge. If there is no major chord in the middle wedge but there is a minor chord, the song is probably in the minor (Aeolian) key.

If the song ends on a chord, it’s a strong clue, and that chord probably signifies the key.

Use your ears! At the end of the song, which note sounds the most “at rest?” That’s likely the root of the home chord.

In this case, based on the ending chord and my ears, “Perfect” is definitely in G major (Ionian).

Step 4: Check for non-diatonic chords

If any chords fall outside the three-position cluster, the song might include modulation, a secondary dominant, or modal interchange.

Sometimes, a song will modulate (change to another key) for the bridge, and sometimes, a lazy songwriter (or producer) will repeat the chorus modulated up a whole step (the infamous “truck driver’s gear shift”). In many of those situations, you can perform separate analyses for each section of the song. Often, in these cases, the verse fits nicely into one key and the chorus another.

If you see a dominant 7 chord that isn’t in the right-hand part of the wedge, that’s likely a secondary dominant, and you can probably ignore it completely for purposes of determining the key of the section as a whole. However, if you are improvising over it, it can be worth treating the dominant 7 chord and the chord it resolves to as a brief modulation and switch scales for it.

Modal interchange occurs when a songwriter borrows a chord from a parallel mode (e.g., C major borrowing a chord from C minor). Radiohead’s “Creep” serves as an example of this. The chords are G - B - C - Cm, and I perceive G as the home chord. From that perspective, the expected chords for that key are G, C, D, Am, Em, and Bm; thus, G and C are expected, while B could be seen as a borrowed chord for Bm and Cm as a borrowed chord for C. I’m not entirely convinced by that, but it does sound cool. My suggestion when you don’t grasp how a chord fits into a key is to try temporarily shifting to the corresponding major or minor pentatonic scale of the new chord. We’ll see examples of this later in “Little Wing” and “Take Me Home (Country Roads).”

Applying the concept to popular songs

Bad Moon Rising

“Bad Moon Rising” by CCR is a three-chord pop song with just D, G, and A chords.

The verse starts and ends on a D, and the chorus follows the pattern of a major blues in D, so we can be confident that the key is D major (Ionian), and the D major pentatonic scale is a great choice. This is as simple as things get.

With any rock-and-roll song, it can also be worth trying out the minor pentatonic scale that matches the home key, which in this case would be D minor pentatonic. This song is a little too “jingle-jangly” for that to work well for my taste, but your mileage may vary.

Proud Mary

CCR’s “Proud Mary” has a non-diatonic main riff that uses C, A, G, and F6 chords, but once you get into the song, the verse and chorus (and solo section) just contain D, A, and Bm.

D sounds like home, so this song is clearly in D major (Ionian).

You Shook Me All Night Long

AC/DC’s classic “You Shook Me All Night Long” is another classic three-chord rocker, with just G, C, and D chords.

G clearly sounds like the home chord, making the key G major (Ionian), and Angus Young used a mixture of G major and G minor pentatonic scales in his iconic solo.

Highway to Hell

AC/DC’s “Highway to Hell” is another three-chord rocker, using mainly just G, D, and A chords, with an E thrown in leading into the chorus.

The song starts and ends on an A, and the chorus begins and ends with A chords, so A feels strongly like the tonal center. Since A is the “right-hand” slice of the main three-chord wedge, this suggests that the song is in A Mixolydian. In this case, the E chord is acting as a secondary dominant because it immediately resolves down a fifth to the home A chord.

Take Me Home (Country Roads)

John Denver’s “Take Me Home (Country Roads)” is mainly composed of A, F#m, E, and D chords, with a G chord in the bridge.

A sounds like home, so the song is clearly in the key of A major (Ionian). The G chord that occurs in the bridge is a bVII chord, which is borrowed from the A Mixolydian mode.

Little Wing

Jimi Hendrix’s “Little Wing” is a contender for my all-time favorite chord progression. It contains seven chords, one for each letter name: Am, Bm, C, D, Em, F, and G.

Six of the chords fit nicely into a wedge, and Em sounds like “home,” so this song is clearly in the key of E minor (Aeolian). The F chord functions as a bII chord (a major chord built one semitone above the E root), which in Classical music theory is called a “Neapolitan” chord (the more you know, right?).

For this song, the Em pentatonic and E Aeolian scales are a great place to start, but when you come to the F chord, you’ll need to adjust. F major pentatonic is a solid choice (and it’s what Hendrix used), but it’s a fleeting moment, so you can also just target one of the notes from the F triad.

Comfortably Numb

Pink Floyd’s “Comfortably Numb” has verse and chorus sections with very different sounds. The verse (and the second guitar solo) are in Bm (Aeolian), using the chords Bm, A, G, and Em, and David Gilmour’s solo leans very heavily on the B minor pentatonic scale.

The chorus (and first guitar solo) alternates pairs of chords D - A and then C - G, making for a picture that’s harder to interpret:

The beginning of the chorus and the first guitar solo (with the chords D - A - D - A) sounds strongly like D major (Ionian), which makes sense since it’s the relative major of B minor from the verse. Gilmour uses all the notes from the D major scale during this section of his solo, which works nicely.

When the chords change to C - G - C - G, this is a modulation to G major (Ionian), and Gilmour switches scales for this part of his solo to reflect this.

The fretboard approach: visualizing keys with root notes

For many songs, you don’t need to draw the cycle of fifths to figure out the key because the cycle of fifths (fourths, really) lives right on your fretboard. Indeed, if the chords of a song all sit inside one “wedge” on the cycle of fifths, they’ll show up as two vertical columns on the fretboard, one for the major roots and one three frets lower for the minor roots.

Here are the chord roots for “Little Wing,” with the minor chords lined up at the 7th fret and the major chords lined up at the 10th:

Notice how the B - E - A and D - G - C notes are lined up just like the roots in the corresponding wedge of the cycle of fifths?

When you use this technique, you will have to be aware of the effect of the warp, so I recommend performing this analysis on the lowest four strings.

Conclusion

I hope you find this technique useful. While some songs may not lend themselves to this level of analysis, they are the exception rather than the norm. The vast majority of songs we play closely adhere to this, and this method will provide you with a solid understanding of what scales you can use to solo over them.

If you’re a Gold Level member, be sure to check out Module 5 of the Improv 101 course. The method I shared there for shifting pentatonic scales to match chords in a progression is intimately tied to this.