Key finders disagree because many songs fit more than one key. A major key and its relative minor share every note, songs borrow chords from other keys or sit in a mode, some change key halfway through, and some are tuned away from A440. Each detector still has to print one answer. To check a reading, open the Key finder (also at key.aden.space), look at the key over time on the waveform, and play the key's chords on its piano against the song.

Most of the time the two answers are not random. They are close relatives, and knowing which relative you are looking at tells you which one to trust.

Why does one key finder say A minor and another C major?

Because A minor and C major use the same seven notes. A key detector measures which notes sound, and how strongly, then compares that profile with the 24 major and minor keys. A key and its relative have nearly the same profile. The real difference is which note feels like home, and that depends on the bass, the chord a phrase resolves to and where the melody lands.

Take a loop of Am, F, C and G. If the phrases come to rest on Am, it is A minor. If the vocal lands on C at the end of every line, plenty of listeners hear C major. Both answers describe the same notes.

For mixing, this disagreement costs you nothing. Relative keys share a Camelot number (8A and 8B) and mix perfectly. The Camelot wheel guide has the full table.

The near misses: fifths and parallel keys

Two other mix-ups come up again and again.

• A fifth apart. A minor and E minor differ by a single note, F against F#. When a song spends most of its time on the notes they share, a detector can tip either way. On the wheel they are neighbours, 8A and 9A.
• Parallel keys. A major and A minor share a root but not their third, sixth and seventh. Blues, rock and soul often blur the two, with a minor third sung over major chords.

Research on key detection treats these as partial errors rather than plain misses. The MIREX evaluation, a long-running benchmark for music analysis algorithms, scores the exact key as 1, a key a fifth away as 0.5, the relative key as 0.3 and the parallel key as 0.2. A scoring system only needs those categories because the misses are so often near ones.

Modes and borrowed chords

Not every song is plain major or minor. A song in D Dorian uses the notes of C major but treats D as home: a loop of Dm and G is a typical example. A song in G Mixolydian uses the same notes with G as home, as in a G, F, C loop. A detector that knows only major and minor keys has to call these something, and C major, D minor and G major are all reasonable guesses.

Borrowed chords pull the same way. A song in C major that borrows F minor or Bb from C minor adds Ab and Bb to the mix of notes, which can pull a detector towards C minor. Many minor songs do the opposite and use a major V chord, E major in A minor, which adds a G#. That is still A minor, but the notes no longer match any single scale.

Key changes in the middle of a song

Plenty of songs change key. The classic is a last chorus that moves up a semitone or a tone, but a song can also move between a minor verse and a major chorus, or sit in one key for the intro and another for the drop. A single key for the whole track is then an average, and two tools that heard different parts of the song can both be right about what they heard.

It also explains why a song can read one way from a link and another from the file. The Key finder reads a song found by name or link from half a minute of it, and a dropped file in full. If that half minute and the rest of the song sit in different keys, the two readings differ.

The Key finder shows a key change instead of averaging it away. A dropped track's waveform is coloured by key over time, and each key is printed on the clip's top strip where it starts. Press one to fit from that key, from that point in the track.

Tuning that is not at A440

Most music today is tuned so the A above middle C is 440 Hz. Not all of it. A recording tuned to 432 Hz sits about a third of a semitone flat. Tape that ran slightly fast or slow, or an old synth that drifted, puts a song somewhere between two keys, and a song 50 cents off sits exactly halfway. Two detectors can round that different ways.

Sped-up and slowed-down edits do the same thing on a bigger scale. About 6% faster without key lock is one semitone higher, so the sped-up edit of a song in A minor reads Bb minor, and a speed that isn't a whole number of semitones lands between keys.

If a song sounds slightly out of tune against the Key finder's piano in either key, the recording is probably off A440. The Key finder pitches in whole semitones, so pick the key that sounds closer and expect a slight detune.

Half time and double time

BPM finders disagree in the same way. A hip-hop beat at 70 BPM can be counted at 140, and a drum and bass track at 174 BPM feels like 87 to someone nodding along to the snare. Both numbers describe the same song; a detector, or a person tapping, picks one level of the beat. When two readings differ by exactly a factor of two, they are the same tempo counted differently.

That matters when you match two songs. The Key finder's Match allows for it: an acapella at 70 BPM over a 140 BPM beat is fitted at double time (×2) rather than played twice as fast. More in how to find the BPM of a song.

People disagree too

Ask three musicians the key of a song and you can get three answers. One goes by the chord chart, one by the bass, one by where the melody comes home. And many key databases fill their numbers from automatic detection, so two websites can simply be repeating two different algorithms. No source is the final word. Your ear, with the song playing, is.

How the Key finder handles this

• Key over time. A dropped track's waveform is coloured by key over time, and each key change is printed where it starts, so one average key doesn't hide two.
• Steady while listening. When it listens through the microphone or a browser tab, it reads the last twelve seconds again every second, and a key only changes once it holds for two readings. A passing chord doesn't move it.
• Relatives count as a match. Pitching keeps the mode and aims for the target's relative key, and in Match, keys that already mix on the wheel can make a great match. An A minor or C major disagreement changes nothing for a mashup.
• Half and double time. Match fits tempos at half or double time when that fits better than stretching.
• Chords. For anything up to a minute, such as a loop or a sample, it reads the chords and lines them up under the waveform.

It still prints one key per stretch of the song, and it can still be wrong. That is why the piano and the chords sit on the same page.

How to check a key by ear

1. Open the Key finder and set the key it read, or the one you are unsure about.
2. Turn on Piano and Chords at the top. Chords lists the seven chords of the key: I ii iii IV V vi vii° in a major key, i ii° III iv v VI VII in a minor one.
3. Play the song and press the first chord, I or i. If it sounds like home, you have the right root.
4. Press the relative's chord: vi in a major key, III in a minor one. If that feels more like home, the song is in the relative key.
5. Press notes on the piano while the song plays. A note that keeps clashing points to a neighbour on the wheel, a mode or a borrowed chord.

For the full method, by ear or with a file, a name or a link, see how to find the key of a song. In ChatGPT or Claude, add Key finder by Aden (mcp.aden.space/key-finder) to ask the same questions in a chat.

FAQ

Why do two key finders give different keys for the same song?

Most disagreements are between close relatives: a major key and its relative minor, which share every note, keys a fifth apart, or a major and a minor key on the same root. Songs that change key, borrow chords, use modes or are tuned off A440 make it harder still, and each tool has to print one answer.

Is A minor the same as C major?

They use the same seven notes but a different home note. A song in A minor resolves to A, a song in C major to C. For DJing and mashups the difference rarely matters, because relative keys mix perfectly.

Which key is right when two tools disagree?

Play the first chord of each key along with the song. The key whose chord sounds like home is the right one. If neither does, check whether the song changes key or sits between keys because of its tuning.

Why do BPM finders give half or double the tempo?

A beat can be counted at more than one level, so 70 and 140 BPM, or 87 and 174, describe the same song. Detectors and people tapping each pick one level. When two readings differ by exactly a factor of two, they are the same tempo counted differently.