Find the key of a song

First it finds the song's tuning: where its notes actually sit compared with A = 440 Hz, since plenty of records are a few cents sharp or flat. Then it measures how strongly each of the twelve notes sounds through the whole song, counting a note only where it stands out from its neighbours, so a loud bass or a bright synth doesn't drown out the harmony.

That twelve-note picture is compared with how each of the 24 major and minor keys typically sounds, as music theory describes it and as measured on over a thousand songs with known keys, and the closest match is the key.

DJs mix by the Camelot wheel: each key gets a number from 1 to 12 and a letter, A for minor and B for major. Keys next to each other on the wheel share all but one note, so they blend without clashing. From 8A (A minor) you can go to 7A, 9A or 8B (C major); the page lists the neighbours of every song's key.

• A minor and C major use exactly the same notes, as does every minor key and its relative major. Which one a song is in depends on where it comes to rest, and that's the hardest call for any key finder. The page always shows the relative key too.
• A key a fifth away (C and G major) differs by one note, and is the next most common confusion. When two keys score almost the same, the page says so.
• Songs that change key are given their main one, the key they spend the most time in.

The page reports how far the song's tuning is from A = 440 Hz, in cents (a hundredth of a semitone). A song tuned to A = 432 Hz shows as about 32 cents flat, still in its key. If you play along, tune your instrument by the same amount, or the key will sound right but every note slightly off.