Roman numerals: talking about music in any key

Open any bouzouki tutorial, any chord chart, any guitar lesson, and sooner or later you’ll see something like this:

I – IV – V – I

That’s a chord progression — but the strange thing is, the symbols aren’t note names. They’re Roman numerals, and they don’t tell you which chords to play. Not directly.

They tell you something more useful: the function each chord plays in the key. Once you understand that, you can transpose the same progression into any key without changing the numerals.

What the numerals mean

Roman numerals number the seven notes of a scale, one through seven. If you’re in the key of C Do major (a scale we’ll formally build in chapter 4), the seven notes are:

C – D – E – F – G – A – B

And the corresponding Roman numerals are:

I – ii – iii – IV – V – vi – vii°

So in the key of C Do :

Numeral	Note	Chord
I	C Do	C major
ii	D Re	D minor
iii	E Mi	E minor
IV	F Fa	F major
V	G Sol	G major
vi	A La	A minor
vii°	B Si	B diminished

A progression written as I – IV – V in the key of C means C major, F major, G major.

But the magic of Roman numerals is this: that same progression in the key of G Sol means G major, C major, D major. Same shape, different notes. The numerals describe the pattern, not the specific chords.

The case rule

Look closely at the seven numerals above:

I – ii – iii – IV – V – vi – vii°

Three are uppercase (I, IV, V) and four are lowercase (ii, iii, vi, vii). That’s not stylistic. It carries meaning:

• UPPERCASE = major chord
• lowercase = minor chord
• lowercase + ° = diminished chord

So I is one-major, ii is two-minor, V is five-major, vii° is seven-diminished. The case tells you the chord quality without you having to look it up.

Why this matters

A song in C major and the same song transposed to G major sound identical in their relationships — only the absolute pitches change. Roman numerals capture the relationship and ignore the specific pitches.

This is why I – IV – V, ii – V – I, and I – vi – IV – V are the shorthand of choice for musicians worldwide. You can hand a guitarist a slip of paper that says I – V – vi – IV in D, and they’ll know exactly what to play — even though those four chords would be completely different in any other key.

A note on the brief

The original sketch for this site mentioned the sequence I – III – IV – I — with an uppercase III. In strict Western functional harmony, that would mean a major chord on the third degree, which doesn’t naturally exist in a major key (the iii chord is normally minor). It’s most likely a typo for I – iii – IV – I.

But there’s an honest exception: some pedagogical traditions — jazz, certain folk analyses, and some Greek δρόμος descriptions — use all-uppercase Roman numerals as neutral degree markers, without encoding chord quality. In those contexts, “III” just means “the chord built on the third degree, whatever quality that turns out to be.”

For this site, we’ll always follow the case convention: uppercase = major, lowercase = minor, lowercase + ° = diminished. It’s the most widely used standard and removes ambiguity.

Recap

• Roman numerals (I, ii, iii, IV, V, vi, vii°) name the seven scale degrees of any major key.
• Uppercase numerals indicate major chords; lowercase indicates minor; lowercase + ° indicates diminished.
• Roman numerals describe the shape of a progression, not the specific chords — meaning the same progression works in any key.
• The pattern for a major key is always I ii iii IV V vi vii°. Memorize this one row.