There are seven chords in every key. If a song is to be thought of as being in a key, then the chords used in that song are chosen from those seven chords.
For instance, the chords (triads) in the key of C major are:
C major, D minor, E minor, F major, G major, A minor and B minor flat 5 (also known as B diminished), these chords are created by stacking notes in 3rds, that is to say from the starting pitch ex. C, you go C then 3rd above C is E then 3rd above E is G so CEG. Another way of saying this is to use the 1st, 3rd and 5th note from a scale. ex. CDEFG 1-3-5 = CEG
If the song is in the key of C major, then any of the seven chords could be present. Everything usually gravitates toward the C major chord (tonic chord). It is not unusual to start and end the piece on the tonic chord.
There are seven possible scales in a single major scale because any one of the seven notes of that scale could be considered as a starting point to create a new scale. The possibilities in the C major scale are:
CDEFGABC DEFGABCD EFGABCDE FGABCDEF GABCDEFG ABCDEFGA BCDEFGAB
These new scales are called modes.
They are named Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian.
Each new scale or mode directly corresponds with the chord that occurs on the same step of the scale.
Each of the modal scales work in the same way, everything will gravitate toward the new starting point on each mode.
Each mode has its own unique characteristic sound and mood, Many styles rely heavily on specific modes.
To be considered to be in a specific mode, the chord of that mode must be emphasized. Playing one of the seven chords for any length of time causes the ear to perceive that chord as a new key or tonal center.
There are three modes that have a major sound, and four modes that have a minor sound.
MODES & CHORDS derived from the C Major Scale
Ionian – CDEFGABC (I) C Major – CEG
Dorian – DEFGABCD (ii) D minor – DFA
Phrygian – EFGABCDE (iii) E minor – EGB
Lydian – FGABCDEF (IV) F Major – FAC
Mixolydian – GABCDEFG (V) G Major – GBD
Aeolian – ABCDEFGA (vi) A minor – ACE
Locrian – BCDEFGAB (vii) B minor b5 – BDF (B diminished)
A typical example is the relative minor. In the key of C the relative minor is A minor which means the potential chords for the key of A minor are the same as the chords for C.
The only difference between the two “keys” is that in the key of C major, everything gravitates around the C major chord, whereas in the key of A minor, everything gravitates around the A minor chord.