Question

Relative major and minor, and why detectors confuse them.

A key detector that says A minor when your track is plainly in C major is not wrong about the notes. It is undecided about the center. Relative major and minor are built from exactly the same seven notes, so nothing in the pitch content tells them apart, and the only thing that does is which note the music treats as home.

What relative major and minor actually are

Every major key has one minor key built from the same seven notes, and it starts on the sixth degree of the major scale.

C major runs C, D, E, F, G, A, B. Start the same seven notes on A instead and you get A, B, C, D, E, F, G, which is A minor. Nothing was added, nothing was taken away, and nothing was sharpened or flattened.

That is the whole relationship. Two names, one set of notes, three semitones apart.

Why this is the most common disagreement in key detection

A detector listens to audio and works out how much of each of the twelve pitches is present over time. That profile is what it compares against what it knows about keys.

For a relative pair, the profile is the same profile. The evidence that would separate them is not pitch content at all, it is emphasis: which chord starts the section, which chord ends it, what the bass sits on, and where the melody comes to rest.

Emphasis is much harder to measure than pitch, and it is genuinely ambiguous in a lot of music. A loop that vamps between the two chords forever has no answer to give.

What actually decides the center

• The bass. Whatever the low end keeps landing on is usually the root.
• The first and last chord of a section. Sections tend to depart from home and return to it.
• Where the melody rests. The note a vocal holds at the end of a phrase carries more weight than any note it passes through.
• The cadence. A chord that pulls hard into another one names the one it pulls into.
• The drone. A pad, a sustained sub or an open string that never moves settles the question on its own.

The twelve pairs, so you can spot one instantly

If a detector's answer is on this list next to the key you expected, you are looking at this problem and not at a broken tool.

• C major and A minor
• G major and E minor
• D major and B minor
• A major and F# minor
• E major and C# minor
• B major and G# minor
• F# major and D# minor
• Db major and Bb minor
• Ab major and F minor
• Eb major and C minor
• Bb major and G minor
• F major and D minor

How to settle it yourself in about ten seconds

Play the track and hum the note it feels like it wants to end on. Then play that note on a keyboard and hold it against the record.

If it sits still and sounds finished, that is the root. If it sounds like it is waiting for something, try the note three semitones away and hold that one instead.

One of the two will feel like the floor. That is the answer, and no amount of analysis outranks it.

What an honest detector should do about it

The useful behavior is to say which of the two it leans to and how strongly, because a lean of almost nothing and a confident read are different pieces of information.

Most tools flatten that into a single label with no indication of how close the call was. You get one key, printed with the same certainty whether the evidence was overwhelming or a coin flip.

That is the habit worth watching for. A tool that never shows you a close call is not a tool that never has one.

Frequently asked

What is the relative minor of a major key?

The minor key built on the sixth degree of the major scale, three semitones below the major root. The relative minor of C major is A minor.

Why does my key finder say minor when the song sounds major?

Almost always because the two keys share all seven notes and the tool weighed emphasis differently. Check whether the answer is three semitones below what you expected, which is the signature of this exact confusion.

Does it matter which one I use?

For writing, a little: the two names point your ear at different chords as home. For matching a sample to a track, much less, because the notes available to you are the same either way.

Can a song be in both at once?

In practice, yes. Modal vamps and two-chord loops often refuse to commit, and calling one of them wrong is a decision the music never made.