I’ll just lay out my full understanding here. The basic chord palette is provided by the key and its associated key signature. If the key is G major, you know the tonic chord (G major) and that there is one sharp, F#, typical of the key. The primary notes you have to work with, then, are G A B C D E F#. If the key is D minor, the tonic chord is D minor and there is one flat, so its notes are D E F G A Bb C.
You don’t have to actually memorize the key signatures; just understand how to figure them out as needed. A major key has the same notes as its major scale and, starting from the tonic note, goes in a pattern of WWHWWWH, where W is a whole step (two frets) and H is a half step (one fret). Likewise, a minor key has the same notes as its natural minor scale and, starting from the tonic, goes WHWWHWW.
The minor pattern happens to be the same as if you start the major pattern on its 6th note (or you could look at it as, the major pattern is the same as starting the minor pattern on its 3rd note. The consequence is that every major scale has a related minor scale and vice versa, called its “relative” scale. They have the exact same notes, just starting from a different tonic. For G major’s relative minor, just move down three frets to the left, meaning E minor. Its notes are E F# G A B C D; compare that to the notes of G major above. If you are looking for the relative major of a minor key, reverse what you did, going three frets to the right. Another concept you should understand is the “parallel” major and minor keys, which just means the ones based around the same letter note. G major’s parallel minor key is G minor, and G minor’s parallel major key is G major.
So your chord palettes are derived from these notes. The most common are the diatonic chords, which are chords built entirely with the notes from the key’s associated scale (major or natural minor. The diatonic triads, meaning diatonic chords made up of three notes, are formed by taking every other note from your starting note. Let’s take G major again: G A B C D E F#. The first diatonic chord is G B D (taking every other note from the first until you get three.) That happens to be the G major chord, or just G. The second is A C E, which works out to E minor. You can do this until you get 7 diatonic triads (when you get to the end of the seven notes of the scale, you just repeat it at the next higher octave). Each has a role within the key, which we characterize with Roman numerals, capitalized if major and lowercase if minor. The first chord, the tonic chord, is the I chord. The second is the ii chord, and so on.
The 7 diatonic triads in a major key work out to be I – ii – iii – IV – V – vi – viiø (the ø means half-diminished). Memorize this pattern and, if you know the notes of the key, you’ll instantly know the diatonic chords too. We know the seventh note in G major is F#, so we automatically know that the vii chord is F#ø.
Like the minor scale, the diatonic chord pattern of a minor key is the same as if you start on the sixth degree of the major pattern, except we start the numbering at i to reflect the starting point. While that would come out to i – iiø – III – iv – v – VI – VII, you’ll probably find over time that it’s more useful to think of each number relative to how it would be in the parallel major key. Example: C major had no sharps or flats; its notes are C D E F G A B. In comparison, C minor’s notes are C D Eb F G Ab. The difference between the two is that in a minor key, the 3rd, 6th, and 7th notes of its parallel major key are flat. This holds up across every key, and it’s useful to translate this into the chord roles as well, making the diatonic minor key chords i – iiø – bIII – iv – v – bVI – bVII.
By the way, you don’t have to stop with triads. You can build diatonic 7ths by adding a fourth note (again skipping every other one), 9ths, and on. For a major scale, for example, the sevenths come out to Imaj7 – iim7 – iiim7 – IVmaj7 – V7 – vim7 – viiø7.
So those are the diatonic chords of the major and minor keys (and their associated major and natural minor scales). But you can also build chords diatonic to the modes. There are seven modes, but the first and sixth modes have the same notes as the major and natural minor scales (called the Ionian and Aeolian modes), so you already have that covered. And, just as you can think of the minor scale as an alteration of the major (that is, having a flat 3, 6, and 7), it’s useful to think of the modes relative to the major scale:
Ionian: same as major
Dorian: b3 and b7
Aeolian: b3, b6, and b7
Phrygian: b2, b3, b6, and b7
Lorian: b2, b3, b5, and b7
You can also group these by major and minor modes, depending on whether there is a natural or flat 3rd note. The major modes are Lydian, Ionian, and Mixolydian. Compared to the Ionian, the Lydian differs by a #4 and the Mixolydian by a b7. The minor are Dorian, Aeolian, and Phrygian. Compared to the Aeolian, the Dorian differs by a natural 6 (that is, it doesn’t have a flat 6) and the Phrygian differs by a b2. The Locrian mode is diminished; it’s rarely used, so let’s put that to the side for the rest of this.
To build the diatonic chords of each mode, do the same thing you did for major and minor, choosing every other note. Because the Lydian and Mixolydian only differ from the Ionian (the major scale) by one note, they will share several diatonic chords, and the differences will follow from the different note. Talking only triads for simplicity, for the Lydian, the #4 affects only the ii, IV, and viiø chords, which are the ones that have the 4th note that you’ve now made sharp. So Lydian changes the ii chord to a major II chord; the IV chord to a diminished #ivø chord; and the viiø chord to a minor vii chord. Mixolydian has a flat 7, so compared with the major, affects the iii, V, and viiø chords, making them iiiø, v, and bVII. Similarly, the you can do the same thing with the dorian and phrygian by focusing on their one-note difference with the aeolian. The result is the following changes added by each mode.
Compared to major:
Lydian – II, #ivø, and vii
Mixolydian – iiiø, v, and bVII
Compared to natural minor, which has i – iiø – bIII – iv – v – bVI – bVII:
Dorian (natural 6): ii, IV, and viø
Phrygian (flat 2): bII, vø, and bvii.
In addition to the modes, there are two variations of the natural minor scale to know. The first is the harmonic minor, which is the natural minor but with a natural instead of flat 7th note. The second is the melodic minor, which is the harmonic minor plus a raised 6th note instead of a flat 6th. The raised 7th of the harmonic minor changes the III, v, and bVII of minor to III+ (augmented), major V, and viiø. The raised 6th of melodic minor changes the iiø, iv, and bVI to ii, IV, and vi.
So these are the diatonic chords by key and mode. But you are not limited to the key and mode in which you are playing; just the opposite, adding in non-diatonic chords brings flavor to songs. You can “borrow” chords from any parallel scale/mode (that is, rooted in the same note), or from the relative minor. For example, if you are playing in G major, you can borrow from G minor and any of the G modes. You can also borrow from its relative minor, E minor; while you may remember that it has all the same notes (and thus the same diatonic chords), you can borrow from E harmonic minor, which does create some differences. Specifically, the 7th note of a minor scale equates to the 5th note of its relative major. So raising that note of the relative minor (for the harmonic minor) would be like raising the 5th note of the major scale, and what you can borrow is, from the perspective of the major scale, an augmented I+ chord, a major III chord, and a #5ø chord.
If you tally all of these options up, what you get is that you can use, from the roots of all 12 possible notes:
I or i or I+
II or ii or iiø
III, iii, iiiø, or III+
IV or iv
V, v, or vø
#vø or bVI
VI, vi, or viø
VII, vii, or viiø
You are not limited to these, though. You can borrow from any parallel key, including not just locrian (the mode we left out) but modes of the harmonic and melodic minor scales, like the phrygian dominant, or augmented, diminished, and wholetone scales. There are also chord substitutions, which are beyond the scope of this post. And probably a bunch of shit I haven’t even heard of yet.
That said, just because you can and should borrow chords doesn’t mean they’ll necessarily sound good. Some work better than others, or better within certain contexts than others. And you are always fighting the risk of losing the sense of key by borrowing chords that fit more naturally within other keys. That’s why learning from other songs is useful, which is why this blog exists.
Finally, this discussion only concerns chord options that one hears relative to the key – the I chord, V chord, etc. But there are also chords that one hears relative to a specific chord. For example, a passing chord that chromatically pulls you to the next chord, not because of its role relative to the I, but because of how its notes compare to the target chord. Or tonicizations, where a chord other than the tonic is temporarily used as the tonic of its own key, and other chords from that key are brought in. For example, secondary dominant chords.
Standard disclaimer, I make mistakes, I could just be an idiot who is completely wrong, yada yada yada.