The Number System
Roman numerals and the Nashville Number System are how musicians talk about chords without naming a single letter. Learn them and you can transpose any song, and see why its chords work, in any key.
Chords by their job, not their letter
Every major key has the same seven chords in the same order. So instead of naming them by letter — which changes with every key — musicians number them 1 to 7by their step in the scale. The song's shape then stays identical no matter what key you play it in.
Roman numerals
The written version uses Roman numerals. A capital numeral is a major chord, a lowercase one is minor, and ° means diminished:
- I ii iii IV V vi vii°
- In C major: C · Dm · Em · F · G · Am · B°
The Nashville Number System
The Nashville Number System is the same idea with plain numbers (1, 2m, 4, 5, 6m…). Session players use it to chart a song in seconds and change key on the spot. A bandleader simply calls “it's a 1–4–5 in G” and the whole band knows the plan.
Why the numbers are worth knowing
- Transpose instantly.“1 5 6 4” is the four-chord pop song in every key — C G Am F, or G D Em C, or D A Bm G. Same numbers, same feeling, new pitch.
- One shared language. The numbers tell a whole band the plan without naming a single letter.
- You hear the structure. They show which chord is home (I), which pulls hardest back to it (V), and which is the wistful cousin (vi) — the same jobs in any key.
Why some numbers come out minor
The quality of each chord is not chosen — it falls out of the scale. To build the chord on any degree you stack two thirds using only the scale's notes (root, skip one to the 3rd, skip one to the 5th). Whether each third spans three semitones (minor) or four (major) depends on where the scale's half-steps land. In a major scale that makes ii, iii and vi minor and vii° diminished, every time.
See it built step by step in the tool — pick any scale degree and watch the root, third and fifth stack up and the quality fall out. Then use the circle of fifths and a capo to move whole progressions between keys.
Reading only gets you so far — open the interactive fretboard and watch these ideas light up on guitar and piano.
Enter the toolFrequently asked
What are Roman numerals in music?
They name a chord by its step in the key's scale: I ii iii IV V vi vii°. A capital numeral is a major chord, a lowercase one is minor, and ° means diminished. In C major that is C Dm Em F G Am B°.
What is the Nashville Number System?
The same idea with plain numbers (1, 2m, 4, 5, 6m…). Session musicians use it to chart a song in seconds and change key on the spot — a bandleader just calls 'it's a 1–4–5 in G'.
Why is the ii chord (like Dm in C major) minor?
You build it by stacking thirds from the scale: D–F–A. The bottom third D–F is only three semitones (a minor third, because the scale's E–F half-step sits inside it); the top third F–A is four semitones (major). A minor third on the bottom makes a minor chord.
How do the numbers help you transpose?
The numbers are key-independent, so '1 5 6 4' is the same song in every key — C G Am F, or G D Em C, or D A Bm G. Keep the numbers, change the key, and the progression transposes instantly.