determine key from chords

September 10, 2008 6:18 AM

Given a set of guitar chords, I want to know how I can determine what key a song is in.

For example, if I play a song using the chords C - D - G, how can I figure out what key I'm playing in? What if I play Am - G - C - F? Or any other set of chords that sound good together? Is there a trick or calculation that will give me the answer? Does it get harder if I'm using sevenths, minors, ninths, or whatever else?

posted by crapples to media & arts (13 answers total)

This is a good guide to the relationship between key and chords... it shows the process of determining which chords belong to a given key. You're trying to do the reverse of course, but it should still help.
posted by le morte de bea arthur at 6:29 AM on September 10, 2008

Reams of music theory have been written about chord sequences - short answer is it's complicated! But for the layperson, the chord which makes the sequence sound 'finished' - like you could end it there and not feel uncomfortable - is usually the key you're in.
posted by [ixia] at 6:34 AM on September 10, 2008 [1 favorite]

Along with what's already said, on guitar, you should be able to play any 1-4-5 song (e.g. both of your examples) by using the A string as your base chord (the key), the D string as your 4th and the E string as your 5th, all on the same fret. With your first example, those chords would put you on the 10th fret, making G your key. In your second example, the 3rd fret, making C your key. Most songs you'd be playing on guitar by reading chords will follow this pattern.
posted by scottreynen at 6:57 AM on September 10, 2008 [1 favorite]

1) Learn what chords are present in each key. (Actually, learn what notes are present in each key and from that you can know which chords are present.basic music theory
2) Learn to hear when the chord sequence feels resolved.

In your examples: C - D - G are all present in the keys of G major and E minor. You don't have an Em in your sequence, so presumably the song is in G major. Am - G - C - F would be either C major or A minor. Since you have both a C and an Am in your sequence, you'll need to listen to the song to tell which it seems to resolve on.

As you start to analyze more complicated songs, you'll find a lot of them which modulate from one key to another as they go along. You can tell these because they have chords that can't all be found in the same key. For example, if you had a song which contained C - D - F - G, you would know that it was modulating between the keys of C major and G major. Usually (although not always), these songs will return to the original key by the end of the song, so you can use that as your guide.

Some genres of music "break the rules" of which chords are present in the key. For example, a typical 12-bar blues in C might contain the chords C7, F7, and G7. According to basic music theory, you would expect only the G7 to be present in the key of C. The other two chords should be Cmaj7 and Fmaj7 - but that wouldn't give you the same bluesy sound. The progression will still resolve to a C or C7, which tells you that the song is in C.

Once you're feeling solid with all of the above, you can start learning about songs in different modes (Mixolydian, Phrygian, etc) where the tonal center doesn't really match the sequence of chords that you might expect in a typical major or minor key.
posted by tdismukes at 7:15 AM on September 10, 2008 [2 favorites]

Circle of fifths. If you've got that under your belt, you've got all the answers you need.
posted by tzikeh at 7:24 AM on September 10, 2008

Very generally speaking, you can determine the key by looking at what chords are major and what chords are minor in the chord progression. That, along with which note sounds like the "home" note will give you the key.

When you harmonize the major scale, you end up with a group of chords. Some are major, some are minor. You end up with this:

Root (I): Major
Second (ii): minor
Third (iii): minor
Fourth (IV): Major
Fifth (V): Major
Sixth (vi): minor
Seventh (vii): half diminished

So, for the key of C, you end up with these chords:

C
dm
em
F
G
am
b half diminished (or bm flat 5)

Looking at your second example (Am - G - C - F), you can see that that progression would be in the key of C major. The Am is the six (vi) chord, the G is the five (V) chord, the C is the root, and the F is the four chord (IV). But, to complicate it a little further, its more likely that this progression is actually in A minor, which is the relative minor of C. Mainly because its the first chord in the progression so the Am will have the more "dominant" sound.

There are some tricks you can use when analyzing a chord progression. For example, if you see two major chords that are one step away in a progression, then they must be the 4 and 5 chords. So most likely in a sequence like D / E / f#m, the D and E have to be the 4 and 5 chords and your in the key of A. So taking your first example (C - D - G), you spot the C major and D major which are one step away. They're the 4 and 5 chords so the key is G.

You can use this trick with two minor chords one step away. They'd have to be the 2 and 3 chords.

Of course all of this is just a basic starting point. Most songs stray from this basic formula and "borrow" chords from other keys. Chords can be altered in all kinds of ways (flat the 7th chord, make the 3 chord major, throw in a minor 4 chord for a few beats, etc etc etc...). That first example of C - D - G, could be in the key of D. With the G being the five chord, and the C a flat 7.

The altered chords will make determining the key a little harder, but you can usually spot enough chords that are in the key, along with using your ear to feel the root note to get the key.
posted by eightball at 7:56 AM on September 10, 2008 [3 favorites]

One thing on eightball's explanation - sometimes a song in C will use D *major*(IImaj) to resolve, partway through a progression, to the G (V), which in turn resolves back to C (I).

(First song like that which pops to mind is the chord under "light of for some desert town" in Gram Parsons' Return of the Greivous Angel.)
posted by notsnot at 8:41 AM on September 10, 2008

This can be a complicated question, but I'll try to keep my answer simple. If you're interested in this you'll want to start reading up on music theory. Try the lessons at musictheory.net

The first and last chord in a piece of music is often, but not always, the tonic, or home chord of the key. You can also try using your ears and listening for when the song feels resolved or at rest.

Beyond that, eightball has it for most simple music. You'll need to be able to recognize the notes that make up given scales and then apply the I ii iii IV V vi viidim formula, which tells you what chords will be created by utilizing only the notes of one major scale. Here's another example: the key of A major contains A B C# D E F# G#. Applying the above formula we get A Bm C#m D E F#m G#dim. All of those chords are built exclusively with the notes of the A major scale, so a piece of music in the key of A major that used those chords could be notated without using any accidentals (additional sharps or flats or naturals).

Minor keys can be more complicated. All major scales become minor scales if you rewrite them so they begin with the sixth note. For instance, A major starting on the sixth note goes F# G# A B C# D E, and we call that F# minor. We say that F# minor is the relative minor of A major and vice versa. Relative keys use the same pitches but begin on a different root note. The term for keys that have the same root note but use different pitches is parallel. So F# minor and A major are relative while A major and A minor are parallel. If you find a piece of music that uses the chords of a major key but seems to rest on the sixth chord, chances are it's in the relative minor. Your Am G C F example might fall into this category. There are other ways to tell when a song is in a minor key, but it's more complicated to explain and easier for now to just use your ears.

Things get more involved when a piece of music uses chords besides those made of the notes of a single scale. What notsnot is referring to is the concept of secondary dominants. When you see a major or dominant chord built on the 2nd scale degree and resolving to the 5th, you analyze that not as a II chord but as a V/V chord, pronounced "five of five." That's because it's borrowed from the key of the V chord, and it's the V chord of that key. In C major, C F D7 G7 C would be analyzed I IV V/V V I. You can use a secondary dominant for any chord in the key except for the vii chord. There's also other ways to borrow chords and do things besides the basic I ii iii IV V vi vii, but this is already getting long and complicated. So check out the musictheory.net stuff and let me know if you have any specific questions.
posted by ludwig_van at 9:25 AM on September 10, 2008 [1 favorite]

Just to add onto what I was saying about the 4 and 5 chord above...

I'm guessing maybe you asked this question because you're trying to figure out how to solo over a progression? Here's an "a-ha!" moment I had a while back when I was trying to get my head around this stuff...

Say someone is jamming on just two chords: G to A over and over again. The song Jane Says by Janes Addiction does this. Well if you want to do a solo over those chords you might think "well I'll try a G major scale". Doesn't sound quite right. Must be an A major scale then. Nope, not quite right either. If you treat the G and A as the 4 and 5 chords, you realize you're in the key of D. Play some licks in D major and it starts sounding "right". For a darker, bluesier sound, play the relative minor of D which is B minor. So, you play a B minor blues scale over the G to A chord and it sounds great.
posted by eightball at 9:47 AM on September 10, 2008

eightball, there's no difference between playing a B minor scale and a D major scale over a given chord progression. They're made of the same pitches.
posted by ludwig_van at 9:56 AM on September 10, 2008

yeah, just a way to think about it. For me at least.
posted by eightball at 10:29 AM on September 10, 2008

... (as related to the fretboard)
posted by eightball at 10:31 AM on September 10, 2008

One other thing I should've mentioned -- if you extend the chords from triads into seventh chords, you get this: IM7 iim7 iiim7 IVM7 V7 vim7 viim7b5. m7b5 chords are also known as half-diminished chords.

So if you saw CM7 FM7 G7 CM7, that'd be a regular I IV V I in the key of C, with no outside notes. But if you saw, for instance, C7 FM7 G7 CM7, that's a V/IV IV V I in the key of C, as the C7 chord uses a Bb, which we see in this case as being borrowed from the key of F.
posted by ludwig_van at 2:53 PM on September 10, 2008