Chord Progression Builder
Render Roman-numeral chord progressions for any major or minor key.
Overview
The chord progression builder turns a string of Roman numerals into real chord names in any key. Type "I V vi IV" and pick C major, and it returns C, G, Am, F — the classic pop progression. Switch to F# major and you'll get F#, C#, D#m, B. The same template works for minor keys, modal progressions, and any extensions you tack onto the numerals.
It's the fastest way to test how a progression sits in different keys for a vocalist's range, to copy a progression you analysed from one song into a different tonal center, or to teach functional harmony without grinding through transposition by hand. Producers also use it to sketch chord beds for new tracks based on tried-and-true Roman patterns.
How it works
Roman numerals encode chord function relative to a tonic. Uppercase numerals (I, IV, V) mean major triads built on those scale degrees; lowercase (ii, iii, vi) mean minor; a degree sign means diminished (vii°); suffixes carry over from chord symbols (V7, ii7, Imaj7). The builder maps each numeral onto its scale degree in the chosen key signature (a major or natural-minor scale derived from the standard 12-TET system) and then spells the resulting chord.
Modulations are handled by treating the new key as a fresh tonic. Modal interchange (borrowed chords like bVII or bVI in major) and secondary dominants (V/V, V/vi) follow the same logic with an extra layer: the slashed numeral retunes the dominant to a non-tonic target.
Examples
Key: C major, Numerals: I V vi IV → C G Am F
Key: A minor, Numerals: i bVII bVI V → Am G F E
Key: D major, Numerals: IV V iii vi → G A F#m Bm
Key: G major, Numerals: I V/vi vi IV → G E7 Em C
FAQ
Why are some numerals uppercase and others lowercase?
Capitalisation indicates quality. In a major key the diatonic chords are I, ii, iii, IV, V, vi, vii° — capitals for major, lowercase for minor, and the degree sign for diminished.
How do I write a borrowed chord?
Use a flat or sharp accidental before the numeral. bVII in C major means Bb major (borrowed from C minor); #iv° in C means F#°.
What's a secondary dominant like V/V?
A dominant chord that resolves to a non-tonic chord. V/V in C major is D7 (the dominant of G, which is V). It briefly tonicises the second chord, adding momentum.
Can the same numeral string work in minor keys?
Yes, but the diatonic qualities flip: in minor the pattern is i, ii°, bIII, iv, v (or V), bVI, bVII. The tool re-evaluates qualities based on the key you choose.
How do extensions translate?
A "V7" stays a dominant seventh in any key; "iiø7" stays a half-diminished seventh; "Imaj7" stays a major seventh on the tonic.