Chord theory
In western music, every key contains seven unique notes. The first note (or “degree”) of that key is the name of the key. Different modes of a key exists, we will focus on the major scale. In the key of C, these are all the white notes on the piano.
| Degree | Name | Note |
|---|---|---|
| 1 | Tonic | C |
| 2 | Supertonic | D |
| 3 | Mediant | E |
| 4 | Subdominant | F |
| 5 | Dominant | G |
| 6 | Submediant | A |
| 7 | Leading tone | B |
In western music, we have the following intervals. All modes use seven of these intervals, so some intervals are not used in that particular mode as is depicted in the table below.
| Interval | Example | Name | Ionian | Dorian | Phrygian | Lydian | Mixo- | Aeolian | Locrian |
|---|---|---|---|---|---|---|---|---|---|
| (major) | lydian | (minor) | |||||||
| 0 | C-C | unison | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | C-Db | minor second | 2 | 2 | |||||
| 2 | C-D | major second | 2 | 2 | 2 | 2 | 2 | ||
| 3 | C-Eb | minor third | 3 | 3 | 3 | 3 | |||
| 4 | C-E | major third | 3 | 3 | 3 | ||||
| 5 | C-F | perfect fourth | 4 | 4 | 4 | 4 | 4 | 4 | |
| 6 | C-F# | augmented fourth | 4 | ||||||
| 6 | C-Gb | diminished fifth | 5 | ||||||
| 7 | C-G | perfect fifth | 5 | 5 | 5 | 5 | 5 | 5 | |
| 8 | C-Ab | minor sixth | 6 | 6 | 6 | ||||
| 9 | C-A | major sixth | 6 | 6 | 6 | 6 | |||
| 10 | C-Bb | minor seventh | 7 | 7 | 7 | 7 | 7 | ||
| 11 | C-B | major seventh | 7 | 7 | |||||
| 12 | C-C | octave | 8 | 8 | 8 | 8 | 8 | 8 | 8 |
There are a couple of tri-tone harmonies that can be made, in the key of C these are the following. The root shall always be the first degree note (or it will be a different chord!)
| Name | Example | Degrees | Intervals | Semitones |
|---|---|---|---|---|
| Major | C | 1-3-5 | M3-P5 | 0-4-7 |
| Augmented | Caug | 1-3-#5 | M3-A5 | 0-4-8 |
| Minor | Cm | 1-b3-5 | M3-P5 | 0-3-7 |
| Diminished | Cdim | 1-b3-b5 | m3-d5 | 0-3-6 |
| Sus2 | Csus2 | 1-2-5 | M2-P5 | 0-2-7 |
| Sus4 | Csus4 | 1-4-5 | P4-P5 | 0-5-7 |
| b2 | Cb2 | 1-b2-5 | m2-P5 | 0-1-7 |
| #4 | C#4 | 1-#4-5 | A4-P5 | 0-6-7 |
A two-tone chord is also possible: this contains the tonic note and the perfect fifth, for example a C5: notes 1 and 5.
A chord can be described with a letter, or with a roman numeral. In the later case, you also need the key to which the chord belongs.
The beauty of using the roman numeral system, is that the chord remains the same after key transpose and it is also more clear what kind of chord you are playing.
In any key, there are seven “normal” chords that use the notes of that particular key. The matrix below give all these chords for the major scale:
| Nr | Chord | Notes |
|---|---|---|
| I | C | 1-3-5 |
| ii | Dm | 2-4-6 |
| iii | Em | 3-5-7 |
| IV | F | 4-6-1 |
| V | G | 5-7-2 |
| vi | Am | 6-1-3 |
| vii° | Bdim | 7-2-4 |
In any mode, you will have seven triad-chords that are in the key of that mode. Always three major chord, three minor chords and one diminished cords. This is depicted below. (as you can see: the “pattern” remains the same, but shifts).
| Mode | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Locrian | I | ii | iii | IV | V | vi | vii° |
| Dorian | i | ii | III | IV | v | vi° | VII |
| Phrygian | i | II | III | iv | v° | VI | vii |
| Lydian | I | II | iii | iv° | V | vi | vii |
| Mixolydian | I | ii | iii° | IV | v | vi | VII |
| Aeolian | i | ii° | III | iv | v | VI | VII |
| Locrian | i° | II | iii | iv | V | VI | vii |
Inversions are ways of playing a chord. Non-inverted chords have the root (or first) note as the lowest note. The first inversion is the chord with the third note of the scale as the lowest note, the second inversion is the chord with the fifth note of the scale as the lowest note.
In a particular key, not all sus chords are possible, as depicted below. The Sus2 has a major second interval between the first note and the sus note, the sus4 has a major second interval between the sus note and the third note (a perfect fifth from the first note). For some sus chords, this note is outside the scale. So in that particular scale, the note is one step lower (for sus2) or one step higher (for sus4).
| Sus2 | Notes | Sus4 | Notes | ||
|---|---|---|---|---|---|
| 1 | C | Csus2 | 1-2-5 | Csus4 | 1-4-5 |
| 2 | D | Dsus2 | 2-3-6 | Dsus4 | 2-5-6 |
| 3 | E | Eb2 | 3-4-7 | Esus4 | 3-6-7 |
| 4 | F | Fsus2 | 4-5-1 | F#4 | 4-7-1 |
| 5 | G | Gsus2 | 5-6-2 | Gsus4 | 5-1-2 |
| 6 | A | Asus2 | 6-7-3 | Asus4 | 6-2-3 |
| 7 | B | Bb2b5 | 7-1-4 | Bsus4b5 | 7-3-4 |
The interesting thing is that the seventh degree of the scale is a diminished chord, so the third note of the chord is not a perfect fifth, but actualy a halve step diminished. So the sus4 is actually a sus4 note, but with the third not a halve step lower (a flat 5)
In the key of C, you don’t have a Esus2, because this would contain a F#, which is not in the key of C. The triad you can make, will have a F note (so a flat-2) You also don’t have a Fsus4, because this would contain a Bb. The triad you can make, will have a B note (so a sharp-4). Remember that this is in relation to the key of the chord, NOT the key of the scale!
A chord is normally made up of the first, third and fifth note of the scale. A seventh Chord also contains the seventh note. The most important seventh chords are:
| Chord | Notes | Intervals | Semitones | Description |
|---|---|---|---|---|
| Major 7th | 1-3-5-7 | M3-P5-M7 | 0-4-7-11 | Major chord with the major seventh note |
| Dominant 7th | 1-3-5-b7 | M3-P5-m7 | 0-4-7-10 | Major chord with a minor seventh note |
| Minor 7th | 1-b3-b7 | m3-P5-m7 | 0-3-7-10 | Minor chord with a minor seventh note |
| Minor-Major 7th | 1-b3-7 | m3-P5-M7 | 0-3-7-11 | Minor chord with a major seventh note |
A sixth-chord is a triad chord with the major sixth note added to the chord. This might be a note outside scale (which might also be true for the seventh chords above).
Additional notes can also be added to the chord, creation “tension” chords, using the major 9th, 11th and 13th note.
Chord substituion is a way of substituting a chord with another chord that is related to the substituted chord. Two approaches are common:
- Substitute a chord from the parallel mode. For example: the fifth chord of a major scale, with the fifth chord of the parallel minor;
- Substitute a chord with the dominant chord of a chord in question. So for example the dominant chord of the second chord of the C major scale. This is notated as V/ii (fifth of second). The Second chord is an Dm chord, the dominant chord of D is A. In most cases, an A7 chord will be used. This is called a borrowed chord. This could also be a another chord from that scale, for example IV/ii (the secondary subdominant, the fourth of second), in this case the fourth cord of D, which is G.