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A piano reference: chords, scales, theory. Full MIDI.

Key Finder

Key: Auto

Play or enter notes

1Note
2Interval
3+Chord
5+Scale or mode

What the Keys tab shows

ColumnMeaning
KeyA key whose scale contains every note you played
NumeralThe Roman numeral of the chord in that key
DegreeWhich scale degree the chord is built on
FunctionTonic, predominant or dominant in that key

How much narrows a key

What you playWhat it settles
One noteEvery key whose scale contains that pitch. Far too many to choose between.
Two notesFewer keys, and the interval between them starts to matter.
One triadA short list. The same chord takes a different numeral in each key on it.
A seventh chordShorter still: a dominant seventh points hard at one tonic.
Several chordsUsually one key. Add them to the progression and the cadence confirms it.

Reference

How many notes does it take to know the key?

More than one chord. A single triad sits inside several keys at once: C major is the tonic of C, the fourth degree of G, and the fifth degree of F, and it belongs to three minor keys besides. That is why the Keys tab lists every key that contains what you played rather than naming one. Adding a second and third chord removes most of them, and a cadence settles the rest.

Why does it not just tell me the key?

Because a single sonority carries no evidence for a tonal centre, and a tool that picks one anyway is guessing in a voice that sounds certain. The Keys tab states what is true of the notes themselves: which keys contain them, what degree they sit on, and what the chord does in each. Naming the likely key is a judgement the music has to earn, usually through a cadence.

Does it find minor keys as well as major?

Yes. Every key that contains the sonority diatonically is listed, major and minor together, each with its own Roman numeral and function. A relative pair such as C major and A minor share all seven notes, so both appear for anything drawn from that scale, and the numeral is what tells them apart: the same chord is I in one and III in the other.

Can I set the key myself instead?

Yes. The Key selector next to the result runs from Auto to any of the 18 keys. Choosing one switches the question from "which keys contain this" to "what does this do in that key", and the Roman numeral and function appear beside the chord name.

To read the chord itself rather than the keys around it, use What Am I Playing?. For the numeral in one chosen key, use the Roman Numeral Analyzer.

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