Chords in a key: what fits together and how to find the key

A key gives you seven chords, one on each degree of its scale. In C major they are C, Dm, Em, F, G, Am and B°, and songs are built out of that handful. Three of them carry most of the weight — the first, fourth and fifth — which is why so much music sounds familiar after two bars.

Write down the chords of the song and look at which key contains all of them. Usually one fits; when two do, the tonic is the chord the song keeps returning to, most often the first and the last. A chord from outside the set is not a mistake — it is a borrowed chord, and it often marks the most interesting bar in the piece.

A melody gives the key away too: count which notes appear and which never do, and the key signature follows. Import a MusicXML or MIDI file and the analysis runs on the whole piece at once, including the places where the key changes halfway through.

C major and A minor use the same seven notes and the same empty key signature — the difference is where the music rests. If the phrases land on C it is major; if they land on Am it is minor. That pair is called relative, and every key has one: it sits three semitones below the major tonic.

A song written for someone else's range rarely fits yours. Move every chord by the same number of semitones and the music stays itself while the top note comes down to where you can sing it. On guitar a capo does the same thing without changing a single shape.

The circle of fifths shows every key and how many sharps or flats it takes. Each chord you find here has its own page in the chord library, each scale in the scale library, and music notation explains how the key signature is written down.