Key finder: what key is this progression in?

A key is a hypothesis, not a lookup

A key is a tonal centre — the note and chord that sound like home — together with the notes and chords that orbit it. Nothing in a list of chords is labelled “home”, so finding the key means weighing evidence. The key finder weighs every major and minor key against what you enter and keeps, for each one, the reasons for and against it. The ranking is those reasons added up: nothing is scored that is not shown to you.

It then says how strong the evidence is: strong, moderate, ambiguous or not enough to tell. These are not percentages — a score is a way to compare keys for one progression, not a probability. A single chord, for instance, cannot establish a key: C is the tonic of C major and also a chord in A minor, G major and F major.

How to find the key from chords

Three kinds of evidence matter, in this order. Belonging: which keys contain the chords, with their qualities — G major and G minor are different evidence. Home: where the progression starts, ends and keeps returning. Resolution: a dominant moving to its tonic. In C – F – G7 – C all four chords belong to C major (I – IV – V7 – I), it begins and ends on C, and F → G7 → C is IV → V7 → I: predominant, then dominant to tonic, with the leading tone resolving, closing the progression. The tool calls that strong evidence. How to find the key of a song walks through the same reasoning by hand.

Why one set of notes can fit two keys

Every major key shares its notes with a relative minor a minor third below: C major and A minor, G major and E minor. Chords built from those notes belong to both. Am – F – C – G reads vi – IV – I – V in C major and i – VI – III – VII in A minor — every chord fits either way, and nothing resolves to either tonic, so the result is ambiguous and both keys are shown. What the music around the chords does — the melody, where phrases land — decides it.

Notes on their own are weaker evidence still. The seven white keys are exactly the notes of C major and A minor; a set of notes has no order and no last chord, so it cannot show which one feels like home.

Why dominant → tonic motion matters

The strongest single sign of a key is its dominant resolving to its tonic. A dominant seventh like G7 contains the key’s leading tone (B) and fourth degree (F), a tritone that pulls inward onto C and E — and G7 belongs to only one major key, so G7 → C names C major in a way no single chord can. A plain G → C is weaker evidence: the same two chords are also I → IV in G major. The tool counts a resolution only when the tonic actually follows the dominant, and counts more when it closes the progression. Cadences covers the different endings.

Why minor keys raise the seventh

A minor key’s natural seventh is a whole tone below the tonic, so its v chord has no leading tone and pulls weakly. Minor-key music raises that seventh — G♯ in A minor — to build a major V. That is why Am – Dm – E7 – Am is strong evidence for A minor rather than against it: its G♯ is the raised seventh, the leading tone that pulls to A. C major can explain the same E7 only as V7/vi, a chord pointing at Am, and its own tonic chord, C, never sounds.

Chromatic chords do not automatically mean a key change

A chord from outside the key usually has a job inside it. Secondary dominants borrow the pull of a dominant to point at another chord of the key: in C – A7 – Dm – G7 – C, A7 is V7/ii: it resolves to Dm (ii), so it points within C major rather than away from it. The tool credits that only when the chord it points at actually follows. Borrowed chords come from the parallel key instead: in C – A♭ – B♭ – C, A♭ is ♭VI, borrowed from C minor: explained, but not one of C major’s own chords. A borrowed chord is colour, not a change of key: it costs a key a little; a chord no reading explains costs it a lot.

Why spelling matters

F♯ major and G♭ major sound the same and are written differently, so the spelling of the chords is evidence for which one is meant. F♯ – B – C♯7 – F♯ is read as F♯ major; G♭ – C♭ – D♭7 – G♭ as G♭ major. Roman numerals are worked out by letter name too, so in C major A♭ is ♭VI while G♯ would be ♯V — the same piano key, a different chord in the key. Enharmonic notes explains why.

What it reads, and what it does not

Chords: the same symbols as the chord transposer — triads, sevenths, sus, 6 and add9 chords, with slash basses. A chord it cannot read is named, never guessed, and the result says it is based only on the chords it understood. Notes: tap them on the piano; black keys can be respelled. It does not listen to recordings or detect a song’s key from audio.

It weighs the 24 major and minor keys. Modal music (a D Dorian vamp, a Mixolydian rock riff) and the blues, whose tonic is itself a dominant-seventh chord, are not modelled as keys of their own, so they come out as ambiguous or as fitting no key well — which is honest, not wrong. A key change partway through is also beyond it: paste one section at a time.

Not sure what a chord is? Name it from its notes with the chord identifier. Know the key? Move the progression with the chord transposer, hear common progressions in any key with the chord progression explorer, see how keys relate on the circle of fifths, or get a full written explanation from the chord progression analyzer. Every key’s chords are listed on its key page, and Roman numerals explains the notation.