Introduction
A key is a group of notes from a particular diatonic (i.e. major or natural minor) scale. A chord is in a particular key if it uses only notes from a particular diatonic scale. For example, chords in the key of C Major will only contain notes from the C Major Scale (only the white notes on piano). Keys are useful in determining which scales and chords fit well together.
Keys
There are seven notes in the Major scale, each of which can be turned into a chord in that key. By taking each note of the Major scale as a root note, and building up in thirds using only the notes of that Major scale, you can construct all the chords in the key of that Major Scale. For example, all the chords in the key of C Major are:
| Name | Tonic | Supertonic | Mediant | Subdominant | Dominant | Submediant | Leading tone |
|---|---|---|---|---|---|---|---|
| Root Note | C | D | E | F | G | A | B |
| Chord | CΔ | Dm | Em | FΔ | GΔ | Am | Bo |
| Notes | C E G | D F A | E G B | F A C | G B D | A C E | B D F |
| Degrees | 1 3 5 | 2 4 6 | 3 5 7 | 4 6 1 | 5 7 2 | 6 1 3 | 7 2 4 |
The reason all of the above chords are in the Key of C Major is because all the notes that make up these chords are found in the C Major Scale (i.e. they have no flats or sharps).
When discussing the degrees of a scale we use Arabic numerals (1, 2, 3, 4…). But when discussing the chords built off a degree of a scale we use Roman numerals (I, ii, iii, IV…). Uppercase Roman numerals (I, IV) denote a Major Chord, while lowercase Roman Numerals denote a minor chord (ii, iii).
Also note the names of each degree. The I is called the Tonic (or Root); the V is called the Dominant. These will be very important going forward.
The above pattern of chord qualities (Major, minor, diminished) is the same for any Major Scale or Key (starting on any root note). That is:
- I = Major
- ii = minor
- iii = minor
- IV = Major
- V = Major
- vi = minor
- vii = diminished
Chord Progressions
So if a Chord Progression uses only the above chords (and ends on the Tonic Chord [i.e. C]) then the entire chord progression is in the key of C Major. For example, take this chord progression:
| Em | Am | Dm | GΔ | CΔ |
All of these chords are found in the Key of C Major, therefore this entire chord progression is in the key of C Major and we can create a melody (or improvise) over this Chord Progression using the C Major Scale. But if we replaced the chord Dm with DΔ (D F# A), then this chord progression would no longer be in the key of C Major, because the note F# (the 3rd of DΔ) is not found in the key of C Major. (Incidentally, this new chord progression would then be in the key of G Major or C Lydian, because of that F#).