Key Finder from Chords — Identify Likely Keys

How does a key finder from chords work?

The parser reduces each recognized symbol to a root plus major, minor, or diminished quality, then scores 24 major and natural-minor candidates. An exact diatonic root-and-quality match earns three points, a diatonic root with another quality earns one, a major dominant in minor earns two additional points, and a tonic root at the beginning or end earns one. Compatibility is the rounded score divided by the maximum available points; it is not a probability.

The parser normalizes recognized A–G chord roots to 12 pitch classes and reduces extensions to a major, minor, or diminished triad quality. It evaluates 12 major and 12 natural-minor candidates: an exact diatonic root-and-quality match earns 3 points, a diatonic root with another quality earns 1, a major dominant in minor adds 2, and a tonic root at either endpoint adds 1. Compatibility is the rounded score divided by three points per chord plus two possible endpoint points. The top three are shown. This is a deterministic fit score—not probability, model confidence, or measured accuracy.

The ranking is deterministic: the same recognized chord sequence produces the same scores. Compatibility describes fit with the documented tables and should not be read as model confidence, measured accuracy or proof of a composer's intended key.

How to get a useful result

Begin with the highest candidate, then compare the alternatives rather than accepting the first label automatically. Play the proposed tonic after the progression and listen for resolution; inspect whether melody and bass emphasize that note; and compare a relative major or minor when the same chords fit both. Roman numerals show exact table matches, while an outside label is a prompt to examine mixture, applied harmony, mode, parsing, or modulation—not evidence that the chord is wrong.

Include a complete, ordered phrase when possible: Enter common chord symbols separated by spaces, commas, bars, or dashes, including enough of the progression to show its resolution. Preserve the cadence and compare close relative-key candidates with the melody, bass and the note that feels like home.

When the top key does not resolve

• If few chords are recognized, separate symbols with spaces, commas, bars, or dashes and use a clear A–G root plus optional accidental.
• If a slash chord or extension gives a surprising result, enter its main triad because the scorer reduces symbols to root and major, minor, or diminished quality.
• If relative major and minor remain close, include the cadence and full phrase, then test both tonic notes with the melody and bass.
• If one section fits and another does not, run them separately to investigate modulation or a temporary tonicization.
• If an intentional borrowed chord appears outside, keep it in the musical analysis; a lower rule-table fit is not proof of an error.

Verify the tonal center with the music

Compare the leading candidates with the progression in order, not only as a bag of chord names. Play the proposed tonic after the final chord, listen for where the bass and melody settle, and inspect the cadence. Relative major and minor keys share many chords, so the musical point of rest often supplies evidence that a table of chord qualities cannot.

Analyze separate sections when a song changes center, and keep borrowed or applied chords visible instead of deleting them merely to improve a score. The compatibility value documents how well the typed symbols fit one major or natural-minor table. It is a reproducible ranking aid, not a probability that replaces harmonic analysis or listening.