Roman numerals

Calling a chord "G major" says which chord it is. Calling it "V in C major" says what it is doing. Roman numerals are the second kind of name.

Case carries the quality

• Upper case = major
• lower case = minor
• ° after the numeral = diminished
• + after the numeral = augmented

So the seven diatonic chords of any major key are always I ii iii IV V vi vii°, and any minor key is i ii° III iv v VI VII.

Sevenths and inversions

Add a 7 for a seventh chord: V7, ii7, IVmaj7.

Inversions are shown with figured-bass numbers, which count intervals above the bass note:

In pop and jazz contexts you will more often see slash notation — G/B rather than V6 — which says the same thing less formally. See chord symbols.

Why it earns its keep

Progressions become portable. I–V–vi–IV is C–G–Am–F in C major and G–D–Em–C in G major. One label, every key.

Patterns become visible. Thousands of songs share a handful of numeral sequences. You cannot see that while everything is written in letters, because the letters differ.

Function becomes explicit. V pulling to I is a relationship between positions, not between the letters G and C. Numerals make the relationship the subject.

What they do not tell you

A numeral gives the chord's root and quality relative to the key. It does not give the voicing, the octave, the rhythm, or which note is on top. Analysis and performance instructions are different jobs.