Roman numerals: the chords behind the numbers

First: a short introduction to Roman numerals

Roman numerals are an older number system also used in music theory. Before analysing any chords, learn three signs: I means 1, V means 5 and X means 10. Combine them to write other numbers.

A smaller sign before a larger one is subtracted: IV = 4 and IX = 9. After it, the smaller sign is added: VI = 6. In ordinary functional harmony we mainly use I to VII, because we build chords on the seven scale degrees.

The chord name tells you what; the numeral tells you where

Take C major: C–D–E–F–G–A–B. Build a triad on each degree by taking alternate notes. C–E–G gives C major, D–F–A gives D minor, and so on.

A chord name such as G identifies the notes forming the chord. A Roman numeral such as V locates the chord’s root within the key being analysed. G is V in C major but I in G major. That is why the system is useful.

Uppercase and lowercase matter

We use an important convention: uppercase numerals indicate major triads (I, IV, V), while lowercase numerals indicate minor triads (ii, iii, vi). A small circle, °, means diminished: vii°. A plus sign, +, indicates an augmented triad, for example III+.

Thus II and ii are different. Both have their root on degree two, but II is major and ii is minor. In C major, Dm is the diatonic ii. D major may appear as II, but contains F♯ and lies outside the key’s usual diatonic triads. We would often investigate whether it has a particular harmonic function.

Move the progression; keep the relationship

Take C–Am–F–G. In C major this is I–vi–IV–V. Now move it to G major, whose notes are G–A–B–C–D–E–F♯. The same degrees give G–Em–C–D. The chord names change, but the numerals remain.

This is transposition in practice. Instead of memorising a song as disconnected chord names, understand the relationships. On guitar choose new shapes; on piano find the new chord tones. Roman numerals provide a shared language.

What do i–iv–V and ii–V–I mean?

Use the numerals to understand two common progressions. In C major, ii–V–I = Dm–G–C, or Dm7–G7–Cmaj7 with sevenths. The roots are on degrees two, five and one. The final chord is home in this key. The numerals describe chord relationships; they do not automatically determine an improvisation mode.

In A minor, i–iv–V = Am–Dm–E. Notice that E is major. A natural minor contains G and would give Em as diatonic v. Raising G to G♯ gives E major a stronger dominant pull. That is why V is common in minor-key harmony. Analyse the actual chord and its musical context.

Play C–F–G–C slowly and notice G’s pull towards C. Then play Dm–G–C. You reach the same home from a different starting point.

Read on: ii–V–I and the pull towards home ↗

Sevenths, inversions and common misreadings

Numerals can show more detail. V7 is a seventh chord on degree five, such as G7 in C major. The 7 describes the chord’s added seventh, not a move to scale degree seven. vii° is a diminished triad on degree seven; the circle is not a zero.

In traditional figured-bass and Roman-numeral analysis, I6 usually means a first-inversion tonic triad, such as C/E. It does not automatically mean a C6 chord with added A. I6/4 means second inversion, C/G. Some modern pop and jazz charts use other systems; check the notation being used.

Similarly, I–IV–V does not tell you to play Ionian, Lydian and Mixolydian in turn. It identifies chords relative to a key. Mode labels and Roman numerals answer different musical questions.

Our method: hear → predict → play → explain → change

Hear: Play I–IV–V–I in C major and listen for the return home. Predict: Name I, IV and V in G major before playing. Play: Check on guitar or piano. Explain: Why is D major V in G major but II in C major? Change: Transpose I–vi–IV–V into another key and write a short melody landing on a chord tone at each change.

This takes you from knowing the signs to using harmony. Roman numerals are not music by themselves; they reveal patterns in what you hear and play.

References: Open Music Theory – Roman Numerals and musictheory.net – Roman Numeral Analysis: Triads. The explanations and practice sequence are adapted for MODUS.