Understanding Harmonic Progressions, Roman Numerals & Cadences

In diatonic music theory, chords built from a major scale are assigned Roman numerals based on their scale degree:

• Uppercase numerals (I, IV, V): Represent Major triads.
• Lowercase numerals (ii, iii, vi): Represent Minor triads.
• Lowercase with degree sign (vii°): Represents a Diminished triad.

In the key of C Major, the diatonic harmonic field is:

By analyzing chords as numbers rather than specific letter names, you can understand their harmonic function and instantly transpose progressions into any musical key. This is the exact vocabulary used in the Nashville Number System, the shorthand working musicians read on stage.

Every diatonic chord falls into one of three core functional categories:

1. Tonic (Home / Rest): I and vi (and weakly iii). These chords provide stability, resolution, and emotional grounding.
2. Subdominant / Pre-Dominant (Movement / Suspense): IV and ii. These chords create forward momentum, guiding the ear away from rest toward tension.
3. Dominant (Maximum Tension / Yearning for Resolution): V (and vii°). Contains the unstable tritone interval between scale degrees 4 and 7, which naturally collapses back into the tonic (scale degrees 3 and 1).

Certain chord sequences form the DNA of contemporary music:

• The Pop Axis (I - V - vi - IV): Used in hundreds of global hits (Let It Be, Don't Stop Believin', Someone Like You). Balances uplifting major lift with melancholy minor depth.
• The Jazz ii - V - I (ii7 - V7 - Imaj7): The supreme cadence of jazz and neo-soul. Features smooth semitone voice leading from the flat 7th of V7 resolving to the major 3rd of Imaj7.
• The Andalusian Cadence (i - VII - VI - V): Classic descending flamenco and rock minor progression (Hit the Road Jack, Sultans of Swing).
• The 50s Doo-Wop (I - vi - IV - V): Nostalgic cycle creating endless cyclical harmonic loops (Stand By Me, Earth Angel).

Great guitar comping is not about jumping disconnected barre chords up and down the neck. It is about Voice Leading—minimizing the physical and pitch distance individual notes travel when changing chords:

• Keep Common Tones: If two chords share a note (e.g., C Major and A minor both share C and E), hold those notes stationary while shifting only the moving voice.
• Stepwise Motion: Move neighboring voices by half-steps or whole-steps whenever possible.
• Guide Tones (3rds and 7ths): Focus on the 3rd and 7th of each chord, which convey 90% of the harmonic color.

To understand exactly how those guide tones are stacked to build each chord, see Guitar Chord Theory.