Find the chords of a song

Every tenth of a second, AudioRinse measures how strongly each of the twelve notes sounds, at the song's own tuning, and separately which note sits lowest, in the bass. It compares that with all 24 major and minor chords: the notes that ring out most are the chord, and the bass note says which of them is the root.

A chord doesn't flicker on and off with every passing note: the most likely sequence of chords across the whole song is chosen at once, so a chord holds until the harmony clearly moves. Then each beat gets the chord heard most in it, and the bar lines go where the changes fall.

We measured it on GuitarSet, 180 recorded guitar accompaniments by six players, against the chords on their lead sheets, scored the standard way: the share of the time it names the right major or minor chord. It got 64% overall, about 81% on the rock and singer-songwriter takes, and 42–60% on jazz, funk and bossa nova, whose chords are mostly sevenths and richer voicings.

With the same player's solo mixed over each take, the way a melody sits over the chords in a song, the rock and singer-songwriter takes scored 70–76%. On 20 full-band songs rendered from sampled instruments (drums, bass, chords and melody) it scored 86%.

So it does best where instruments play the chords outright: strummed guitar, piano, a band. A voice or a lead line louder than the band, or a song carried by a melody alone, gives it less harmony to hear.

Transpose renames every chord up or down by the same number of semitones, for a singer's key or another instrument. The recording itself keeps playing in its own key; to move the sound too, use the transposer.

Capo shows the shapes to play with a capo on that fret: with a capo on the 2nd fret, a song in A minor is played with G minor shapes. The chords sound the same; only the fingering changes.

• Guitar, ukulele and piano players learning a song by ear, or checking a chord sheet they found.
• Bands and worship teams writing out a song's chart before rehearsal.
• Songwriters naming the chords of a demo or a voice memo they recorded.
• Singers who want the chords in their own key: transpose them, then move the song itself to practise.