Roman Numerals to Piano Chords: Analyse and Change Key

What Roman Numerals Show Pianists

Chord symbols such as C, Am, F, and G tell you the notes to play in one specific key. Roman numerals describe each chord’s role relative to a tonic. In a major key, the basic diatonic pattern is:

I ii iii IV V vi vii°

Uppercase numerals normally indicate major triads, lowercase numerals minor triads, and the degree sign indicates a diminished triad. In C major, this gives I = C major, ii = D minor, iii = E minor, IV = F major, V = G major, vi = A minor, and vii° = B diminished.

This is useful at the piano because the labels survive a key change. The progression C–Am–F–G is I–vi–IV–V in C major. In G major, I–vi–IV–V becomes G–Em–C–D. The chord names change, but the tonic departure, relative-minor colour, predominant motion, and dominant return are retained.

Roman numerals are an analysis method, not a claim that every progression has one unquestionable label. Context, melody, bass, inversions, borrowed chords, and modulation can affect the best reading. Start with the clearest local tonic and the chord roots before adding finer detail.

A Reliable Method for Major-Key Changes

1. Identify the likely tonic. Look for the point of rest, a final chord, repeated cadences, and the scale used by the melody. Do not assume the first chord is I.
2. Write the source major scale. For C major: C D E F G A B. For G major: G A B C D E F♯.
3. Find every chord root’s scale degree. In C major, A is degree 6, so an A minor chord is vi.
4. Check chord quality. The diatonic major-key qualities are fixed as I major, ii minor, iii minor, IV major, V major, vi minor, vii° diminished.
5. Write the destination scale. Assign each Roman numeral to that scale degree and use the correct diatonic quality.
6. Voice the result for piano. Keep common tones where possible and move the other voices by small intervals. A correct transposition can still sound awkward if every chord is placed in a distant register.

For a quick manual reference, a chord-transposition aid may help with checking named chords, but it does not replace deciding the key and harmonic function. A compatible notation editor is the appropriate place to enter a progression when you need formal notation beyond a manual chord chart.

Worked Example: C Major to E♭ Major

Here is an original eight-bar teaching progression in C major:

| C | Am | F | G | Em | Am | Dm G | C |

First, analyse it against C major:

| I | vi | IV | V | iii | vi | ii V | I |

Now change the key to E♭ major. Write the scale: E♭, F, G, A♭, B♭, C, D. Its diatonic triads are E♭ major (I), F minor (ii), G minor (iii), A♭ major (IV), B♭ major (V), C minor (vi), and D diminished (vii°).

Substitute each numeral:

| E♭ | Cm | A♭ | B♭ | Gm | Cm | Fm B♭ | E♭ |

For a playable left-hand/right-hand sketch, use root notes in the left hand and close-position triads in the right. Begin with E♭ in the bass and E♭–G–B♭ in the right hand. For Cm, retain E♭ and G while moving B♭ up to C; this makes the change smoother than repeatedly jumping to unrelated shapes. On the final Fm–B♭–E♭, listen for the ii–V–I pull toward the new tonic.

The answer is not E♭–A♭ minor–D♭–E♭. That would preserve some alphabetical-looking distances rather than the progression’s functions. In the target key, vi must be C minor, because C is the sixth degree of E♭ major.

Minor Keys and Secondary Dominants Need Extra Care

Minor-key analysis requires stating which form of minor is in use. Natural minor has the scale pattern that produces i, ii°, III, iv, v, VI, and VII. In harmonic minor, the raised seventh degree creates a major V and a diminished vii°. For example, in A harmonic minor (A B C D E F G♯), V is E or E7, leading strongly to Am. Therefore, label a progression’s collection or explain an alteration rather than treating one minor-key quality pattern as universal.

Accidentals may also signal applied harmony. In C major, D major or D7 contains F♯, which is outside C major. It can function as V/V: the dominant of G, and G is V in C. A concise progression is D7–G–C, analysed V/V–V–I. To move that function to E♭ major, identify its V: B♭. The dominant of B♭ is F, so the corresponding progression is F7–B♭–E♭. The seventh on F7 is E♭, correctly spelling the chord F–A–C–E♭.

Do not force every non-diatonic chord into a secondary-dominant label. Borrowed chords, chromatic passing chords, pedal points, and genuine modulations may need different explanations.

Practice Exercise and Answers

Exercise: Analyse D–Bm–G–A in D major, then write it in A major. Next, change A7–D–G from G major into D major while preserving its applied-dominant function.

Answers: D–Bm–G–A is I–vi–IV–V in D major. In A major, it becomes A–F♯m–D–E. In G major, A7–D–G is V/V–V–I, because A7 leads to D. In D major, V is A, and V/V is E7, so the result is E7–A–D.

Play the answers slowly in block chords, then break each chord into a simple accompaniment pattern. Say the numeral before the chord name. This reinforces function rather than relying only on hand shapes. For optional ear-and-hand practice, piano drills with MIDI live input can be used to rehearse chord recognition and response; they are not song import, audio transcription, PDF reading, or automatic Roman-numeral analysis.

Troubleshooting Common Analysis Errors

A Precise AI Assistant Prompt and Its Limitations

Use this prompt when you want help checking a manual analysis:

“Analyse this chord progression for piano: [paste chords]. Assume the home key is [key] unless the chords provide stronger evidence otherwise. List each chord’s notes, Roman numeral, inversion if supplied, and any non-diatonic explanation. Then transpose the functional progression to [target key]. Spell every chord correctly, distinguish natural from harmonic minor where relevant, and show a four-part, close-position voice-leading suggestion. Mark uncertain interpretations instead of guessing.”

An assistant can propose labels and spellings, but the result should be checked against the score, bass line, melody, and what you hear. A chord-only list may omit inversions, rhythm, enharmonic intent, and the passage length needed to confirm a modulation. It also cannot establish a performer’s preferred voicing merely from Roman numerals. Treat the output as a draft for musical verification at the keyboard.

Sources and checking notes

Examples are original practice material. File-format references are linked where relevant; conversion results still need musical checking.

• Open Music Theory: Roman numerals

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• Chord Transposer
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