Nathaniel School of Music

The Roman Numeral System Explained: Build Any Chord Progression by Number

The Roman numeral system names each chord by its scale degree, using capital numerals for major chords, small numerals for minor, a degree sign for diminished and a plus sign for augmented, so a progression like I iii IV V can be played in any of the 12 keys.

Video lesson by Jason Zac · Beginner · 16 min

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Key points

What this lesson teaches

In this theory lesson, Jason Zac explains the Roman numeral chord system, a long-established way of naming the chords in a progression. It works for everything from simple triads to jazz extensions. It suits any pianist or musician who wants to understand chord function and transpose songs quickly.

Why use Roman numerals?

You could write a progression as F, A minor, Bb, C. That tells you what to play, but only in one key. Written as I iii IV V, the same progression can be moved to any of the 12 keys on the fly. This matters in studios, at concerts, when a singer needs a different key, or when you want a key change at the end of a song. Numerals also show the function of each chord.

The intervals behind the numbers

Take C major: C D E F G A B. Against C these are the root, major second, major third, perfect fourth, perfect fifth, major sixth and major seventh, then the octave. The notes outside the scale are:

There is no note between E and F or between B and C. Remember that Roman numerals name chords built on these degrees, not single notes.

Chords of the major scale

In C major the chords are C, Dm, Em, F, G, Am and Bdim. The rule is simple:

So the major scale reads I ii iii IV V vi vii°.

Adding extensions

The numeral stays put while you add the chord quality after it. Write Imaj7 (or I with a small triangle), ii-7 or iim7, iii-7, IVmaj7, V7, vi-7, and for the seventh degree the half-diminished chord, written viiø7 or vii-7b5. Bigger chords work the same way: a V7b9 is still a V chord. In F major, for example, C7b9 is written V7b9.

Borrowed and non-diatonic chords

To bring in chords from outside the key, keep the degree and change the case. In the progression I III IV iv in C, you play C, E major, F, then F minor. The E major and F minor are not in C major, and ending on iv gives a lovely minor plagal cadence.

For chords built on notes outside the major scale, add a flat or sharp before the numeral. I V bVI bVII in C is C, G, Ab, Bb, which loops nicely back to C. The bVI is built on Ab, a minor sixth above C.

The harmonic minor scale

C harmonic minor is C D Eb F G Ab B: the third and sixth are flattened, while the seventh stays natural. Measured against the major scale, its chords are:

  1. i: C minor
  2. ii°: D diminished
  3. bIII+: Eb augmented (Eb G B)
  4. iv: F minor
  5. V: G major
  6. bVI: Ab major
  7. vii°: B diminished

Conventions vary. Some people write III+ and VI when thinking from the harmonic minor itself. Jason Zac prefers to reference the major scale, which is very common. The system is ultimately a tool for your own understanding.

How to practise

Questions about this lesson

Why use Roman numerals instead of chord names?

Chord names only work in one key. Roman numerals describe each chord's position in the scale, so you can transpose instantly when a singer or bandmate needs a different key.

What are the Roman numerals for the major scale?

They are I ii iii IV V vi vii°. Capitals are major chords, small numerals are minor chords and the degree sign marks the diminished seventh chord.

How do you write a chord that is not in the key?

If the root is in the scale but the quality changes, switch the case, such as III for E major or iv for F minor in C. If the root is outside the scale, add a flat or sharp, such as bVI for Ab major in C.

What are the Roman numerals for the harmonic minor scale?

Measured against the major scale, they are i ii° bIII+ iv V bVI vii°. Some musicians write III+ and VI instead, because conventions vary.

Practise it

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