We can use roman numeral analysis to number the diatonic chords of the major scale. These chords are so commonly used, musicians can specify something like the 'one' chord: which would be a reference to the root/home chord of the key we're in. Continuing on this example, if someone refers to the 'five chord in the key of C', that means we're talking about the chord that is built on top of the fifth scale degree of C major, which is G.

A common way to communicate a chord progression to another musician would be stating the root chords like 'six - two - five -one'.

VI - II - V - I

flowchart TD

  

subgraph Z[" "]

direction LR
classDef grey stroke:#17171f

A[A min]:::grey ~~~ D[D min]

D[D min]:::grey ~~~ G[G maj]

G[G maj]:::grey ~~~ C[C maj]:::grey

end


Z

Some chord progressions are so common, we can easily communicate them using this system. By simply stating 'five', it is assumed we are talking about the 5th diatonic chord of the major scale. In the key of C, that would be a G maj.

Communicating information about minor scale harmony

Similar to the way we can say C to refer to a C major chord but need to state Cm to refer to a C minor, by default we assume the usage of numbered chords to be referring to major scale harmony. We need to implicitly state we're operating in the minor key by using their qualities and including their relation to the corresponding major scale degree: such as ♭iii maj7, which is Eb maj7 in the key of C. Writing minor scale harmony is sometimes done with lowercase roman numerals.

C Natural Minor (Aeolian)

Related information can be found in my posts about modal theory and diatonic/chromatic notes . In summary, standard western music theory is based off and centered around the major scale. We can refer to notes in the key we're in just by referring to their scale degree, or their proximity to a degree. For example, B in the key of C Major would be a major 7th or could even be called a 7th as a shorthand, while the B♭ would be a minor 7th or ♭7. Take a look at this interval chart to get a better idea if you're unfamiliar with how we reference notes in a tonal key. This allows us to talk about purely technical musical ideas without even discussing a key. Being familiar with intervals is crucial to understanding music theory.

This system accommodates complex progressions as well. For example, we can specify something like a ♭II maj7 chord and instantly indicate that we are operating in a Phrygian mode.

C Phrygian

Theres no rules when composing music. It is quite common to borrow a chord from different modes.

ii min7 - ♭ii maj7 - i min

flowchart TD

  

subgraph Z[" "]

direction LR
classDef grey stroke:#17171f

D[D min7]:::grey ~~~ G[D♭ maj7]

G[D♭ maj7]:::grey ~~~ C[C min]:::grey

end


Z