The commonly-used convention for describing Chord Structure is a number system that uses the major scale as the point of reference.
The system works as follows:
- Numbers are assigned to each note in the major scale in ascending order.
- In the key of C: C=1, D=2, E=3, F=4, G=5, A=6, B=7
- The number “1” always represents the root of the chord — always!
- The other numbers correspond to notes in the major scale beginning on the root note.
Example: C Major Triad (chord structure = 1-3-5)…
In addition, chords that include “9ths”, “11ths”, and “13ths” are possible. Let’s add those to the mix in the key of C: D=9, F=11, and A=13…
Example: C Major 9th (structure = 1-3-5-7-9)…
Furthermore, chord tones are not limited to just the natural notes in a major scale. To include all possible notes, any of the numbers can be modified by sharps or flats.
Example: C Minor 7th (chord structure = 1-b3-5-b7)
Using the number system above, you can describe the structure of any chord type.