Roman numeral analysis in jazz

The basic system

A Roman numeral names a chord by its position in a key rather than by its letter. Cmaj7 in C major is I; the same chord in G major is IV. That is the entire point of the notation: it describes a function, so the analysis of a tune survives transposition.

Case carries the quality. Upper case is major or dominant; lower case is minor or diminished. A degree that is not in the key gets an accidental in front of it, written flat-side by convention: bVI, bVII, bII. This holds even inside a minor key, where bIII, bVI and bVII keep their flats although they are diatonic there.

The seven degrees of a major key

• I — major seventh. Tonic.
• ii — minor seventh. Predominant.
• iii — minor seventh. Tonic function, weakly.
• IV — major seventh. Subdominant.
• V7 — dominant seventh. The only one in the key.
• vi — minor seventh. Relative minor; tonic function.
• viiø7 — half-diminished. The dominant without its root.

What the numerals cannot tell you

Roman numerals describe position and quality. They do not describe register, voicing, rhythm, duration or what the melody is doing — and all four of those can overturn a functional reading. A V7 that lasts an eighth note in the middle of a bar and a V7 that occupies two bars are the same numeral and completely different musical events.

They also flatten ambiguity. A chord that could reasonably be read two ways gets one label, and the label then looks like a fact. That is why this analyzer publishes a confidence level with every numeral and shows the alternative reading when there is one: the numeral is a claim, and a claim should come with its evidence.

Run the third example above — Cmaj7 | A7 | Dm7 | Db7 | Cmaj7 — and you get I, V7/ii, ii, subV7, I: two slashed numerals in five chords, and neither of them means the piece left C major.

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