I'm a guitar player and am learning fiddle. I have two questions . . .

  1. If I know the chords of a song, how do I know what key it's in?
  2. Once I know what key it's in, how do I know which notes "fit"?




Generally speaking, a song's key is represented by the one chord where it sounds like it could finish on that chord - often, but not always, the final chord of the whole song, and usually, but not always, the beginning of a verse.

The 3 main chords, diatonically of a major key are I, IV and V. So by determining the 3 main major chords, it's not too tricky to work out which will be I (as IV and V are next to each other, letter-wise)

Bear in mind that many songs will contain non-diatonic chords, and whilst those obviously fit the song, won't be much help being considered when calculating the song's key.

Knowing which notes fit is a lot more complex, but a great start point will be to consider initially the commonest. Those will usually be the scale notes from that key - the diatonic notes, using the key signature that the found key has been given, those notes are easily found.

  • And diatonic notes in a key is your do-re-mi scale? Aug 13 at 13:22
  • Yes, as long as it's not 'fixed do'. Do will have to be moveable, as in do=tonic note.
    – Tim
    Aug 13 at 13:24

It would be easier to explain in terms of specific examples, but I will try to give a greatly simplified general process. There are many, many exceptions.

Your second question is easier to answer: Learn all the key signatures to know which notes are part of each key. Other melody notes will also "fit" at times, but that topic is far too broad for a concise answer here.

As to your first question: First, learn how to construct the chords from their symbols. Then:

  • For each chord in the song, write out the chord's tones.
  • Sort out all the chord tones (pitch classes) in order.
  • If there are fewer than 7, the key can't be determined unambiguously. You will have to look at the pitch classes in the melody. Even then it may still be ambiguous.
  • If there are more than 7, then there are some chords outside the key. Smart money is often on a secondary dominant.
  • If there are exactly 7, compare them to the tones in the various key signatures and see which one fits.
  • If none of them fits, it's hard to generalize what to do.
  • Each major key has a relative minor, so if there are minor chords, you may have two choices based on the pitch classes.
  • Often, the last melody note of the piece will help decide.
  • You may still have it wrong, depending on how the different pitches are used in the song.

Example 1: Chords are D, Em, A7

  • D = D, F♯, A
  • Em = E, G, B
  • A = A, C♯, E, G

Pitches: C♯, D, E, F♯, G, A, B

Key: D Major.

Example 2: Chords are B♭, D7, E♭, F, Gm

  • B♭ = B♭, D, F
  • D7 = D, F♯, A, C
  • E♭ = E♭, G, B♭
  • F = F, A, C
  • Gm = G, B♭, D

Pitches: C, D, E♭, F, F♯, G, A, B♭

Key: The E♭ and B♭ suggest it's B♭ or its relative minor, Gm. The F♯ is the outlier, and D7 that contains it is the dominant for Gm so that's the best bet. Verify by looking at the melody.

This is just a start!

  • This is excellent, thank you! Aug 17 at 17:43

Any chords that contain only notes in a scale are in that scale.

for example, G major is G, A, B, C, D, E, F♯. it contains:

  • G Major - G, B, D
  • A Minor - A, C, E
  • D 7th - D, F♯, A, C

G Major scale does NOT contain the A Major chord (A, C♯, E) because the scale does not include C♯.

here is a tool to help:


The plot thickens as many fiddle tunes use a different set of notes than the major and minor scales we're used to. "Old Joe Clark," for instance, is in Mixolydian mode. That means, if you start on an A and end on a D, that it has an F#, which would make you think of G major, but D is definitely the "home note" that it centers around. Much Appalachian music is pentatonic, meaning that even after you've figured out the "home note," the melody doesn't use all 7 notes in the scale.

You've asked two questions. One is in some ways impractical, a music theory question, "What is the key." The other is more practical, "What notes 'fit'," which I take to mean "If I'm doing a solo, improvising melodically, which notes should I use?"

For practical purposes, when approaching a tune, "what's the key" can be taken to mean "what's the 'home note.'" So if you say "Let's do 'Old Joe Clark," and I say "What key," and you say "D," I'm not going to split hairs about modes but understand that I should start on A so as to end on D (and thus the chords progression will be almost all D all the way through, except for a couple fleeting moments of C). After that point, especially if you already know the chords, asking "what key is this" is a question that helps you better understand what you're playing. Knowing that it centers around D helps explain why there's so dang many D chords. Knowing that it's mixolydian helps explain why that C chord has that strange feel. Knowing that a tune switches from major into minor and back to major might suggest that you alter the mood of your playing in a happy/sad/happy way.

"What notes fit" is an age-old question, especially in jazz circles. The answer is "Well, during which chord?" And the easiest answer is "When you play a wrong one, you'll know." Once again, music theory can help you understand why... but try playing a C# during Old Joe Clark, or a Bb, or an Eb; you'll soon hear it doesn't "fit." Why? Well, the whole notion of a key (or, better, "tonality") is not just that it's a collection of notes (like the solar system is a collection of lumps of rocks and gas), but that they have functions and relate to each other (like the planets exert gravitational pull on each other, and all circle inexorably around the biggest lump of gas). A note outside of that collection is like an alien comet trying to blast through; it won't stick around long and it might disrupt things in the process. On the other hand, jazzers can tell you that once you know the rules you can break them on purpose for effect. The slides and bends of blues are so effective because they tug against (or into) those gravitational pulls. And, as one blues guitarist I know likes to point out, "Any 'wrong note' is only one away from a 'right note.'" If the chord at the moment is D, then you can play a D, F#, or A and be going right along with the chord. But should you accidentally play a B, just follow it with A and call it an ornament. Get totally disoriented and play a C#? Just slide into D and everyone will think you're getting wild.


Provided you know the music is really major/minor key music, the simple thing is to look for either:

  • V7 to I a complete dominant seventh chord to a tonic, diatonically there is only one dominant seventh chord in a key and its tonic is the root a perfect fifth below. ex. G7 is dominant seventh chord, the root a perfect fifth below is the tonic C, labeled I, major or minor will be determined by the key signature
  • or arrange chords by root in decending fifths, ex. V I IV or ii V I (or their minor forms V i iv, and iio V i), i is the tonic.

With either approach you need to learn basic patterns: the diatonic chords on all seven roots, and key signatures. The diatonic chords and Roman numerals are:

major: I IV viio iii vi ii V I

Upper case means major triad, lower is minor, and o is diminished triad. Skipping minor, it isn't a simple formula, you must study it. Notice when the roots are arranged by fifths that in terms of chord quality no three chord quality sequence repeats. There is only one major major major sequence, only one minor major major sequence, etc. etc. This is the essence of finding the tonic with just chord names.

If you told me you had three chords, arranged by fifths, and their qualities were minor major major, I know it fits only the diatonic pattern ii V I, and whatever is the third chord it's the I, it's the tonic.

But, you should also think in terms of finding the tonic melodically, then determine the mode. For example, if A is the tonic, and F# is used throughout, with no other accidentals, the music is likely in dorian mode.

Once I know what key it's in, how do I know which notes "fit"?

In part you're getting into diatonic versus chromatic music.

First learn the diatonic stuff and analyze simple diatonic tunes. Folk song books are a good source to use.

When you have a handle on diatonic music the chromatic stuff will be easier to handle. When you get into chromatic music all twelve tones will "fit", but it's more a matter of how the extra 5 chromatic tones fit it the seven diatonic tones of a key. Keys are primarily diatonic with additional chromatic harmony that usually follows a fairly small number of conventions.

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