This is an important concept that opened a lot of things up for me when I learned it. I’ve alluded to it in other posts, but it’s worth just laying out in one post that we can reference back to.
“Diatonic” refers to the unaltered notes of a particular scale. For example, if you are using the natural major scale, diatonic notes are the notes of the major scale, and diatonic chords are chords that are built purely from the notes of the major scale. This concept can be applied to any scale, allowing you to determine which chords relate to which scales, and which alterations to scales affect which chords. The implications for songwriting and lead guitar over particular chords are, needless to say, massive.
Even better, it is incredibly easy to figure out. Chords are primarily built simply by taking every other note from the scale, starting with the root note. Let’s take the C natural major (C D E F G A B C) and C natural minor (C D Eb F G Ab Bb C) scales for examples. The first diatonic chord of each is rooted in the first note of each scale, C. For C major, skip the D, take the E, skip the F, take the G, and you get C E G, which is the C major triad. For C minor, take the C, skip the D, take the Eb, skip the F, and take the G, and you get C Eb G, which is the C minor triad.
I must add that the notes within a triad are labeled by their position relative to the root. In the C major chord, C is the first note; E is called the third; and G is called the fifth, referencing their position relative to the C in the C major scale. In the C minor chord, the difference is that the “third” is Eb instead of E, referencing its position relative to the C in the C minor scale. (this actually changes when you get to upper extensions like 13ths, but don’t worry about that now).
And you don’t have to stop with triads (three-note chords). Following the same principle, in C major, keep going and skip the A and take the B, and you get C E G B, which is Cmaj7. In C minor, skip the Ab and take the Bb and you get C E G Bb, which is Cm7. In fact, you can continue doing the same thing until you get every note in the scale, because the odd number of notes in a 7-note scale means that starting over an octave shifts the notes you take by one note (and the 2 becomes 9, the 4 becomes 11, and the 6 becomes 13). Go all the way with C major and you get C E G B D F A, which is Cmaj13 (in practice you wouldn’t play all of the notes in the theoretical chord for reasons I won’t get into now). Go all the way with C minor and you get Cm11b13 (See what I meant earlier about the naming conventions for upper extensions? The 13th in a minor is named by its position relative to the major scale, a b13, even though it is the 13th note of the minor scale. Again, no need to worry about that now). Each of those chords is actually the complete C major and C minor scales, respectively, in chord form instead of playing the notes sequentially.
But for now, let’s just stick with triads. While figuring out the tonic chord is certainly useful, we don’t stop there. There are seven notes in each of the major and minor scales, and you can build seven diatonic triads from those notes. Starting with the second note of C major, you get D, F, and A, which is D minor. Start with the E and you get E, G, and B, which is E minor.
You don’t need to memorize these for each key, because the pattern is identical for each one. For the natural major scale, the pattern is major, minor, minor, major, major, minor, diminished. For the natural minor scale, the pattern is the same, but as if you are starting with the sixth chord of the major pattern: minor, diminished, major, minor, minor, major, major.
To generalize across the keys, we use the convention of referring to the chords with Roman numerals, and we capitalize for major chords and use lowercase for minor chords. So the diatonic major scale chords are I, ii, iii, IV, V, vi, and viidim. In C major, that translates to:
For minor and other scales, we slightly alter the naming convention, in that we refer to the chord number relative to the natural major scale. For example, the third diatonic chord of the natural minor scale is a major chord, per the pattern above. The third note in the C natural minor scale, on which the third diatonic chord is rooted, is Eb. Compared to the C major scale, which has a third note of E, the third note of the minor scale is thus flattened, or a “flat 3.” Instead of naming the third diatonic chord of the minor scale a III chord (its position within the minor scale), we name it a flat-III chord, or bIII, referencing how it differs from the major scale. By keeping everything in reference to the same point, the natural major scale, it makes it easier to keep the chords straight when mixing keys and modes and scales. So the minor diatonic chord pattern of minor, diminished, major, minor, minor, major, major translates in C minor to:
And the same holds true with any scale. Take the modes for example. The mixolydian mode is the natural major scale with a flat 7 note. While four of the diatonic chords stay the same, three of them change: the chord rooted at the 7th note (now a flat 7th note; the chord rooted at the third note, because it contains the 7th note when you take every other note to build it (3 – 5 – 7); and the chord rooted at the 5th note, for the same reason (5 – 7 – 2/9). If you tally the notes in the same way as above, in mixolydian, the changes from natural major work out to be a bVII chord instead of viidim, a iiidim chord instead of iii chord, and a v chord instead of V.
Finally, if you take one thing away from this, take this away from it: diatonic doesn’t mean “correct.” Diatonic chords give you the sound of a particular scale, and are a useful reference point for figuring out departures from a scale (as in, this bVII chord means we are using mixolydian here), but you are not limited to diatonic chords when playing in a key or mode. There are great songs built entirely from the diatonic chords of a key, and there are great songs that add non-diatonic chords from other keys and scales. That, in fact, is the purpose of doing the harmonic analyses in this blog: to understand how songs are constructed in this fashion.