Every piece of Western music is built around a scale — a set of notes that sound good together. The key names which scale that is and where it starts. A track "in A minor" uses the notes of the A minor scale as its harmonic foundation. The melody, the chords, the bassline — they all orbit that root note, A, the way planets orbit a star.
The 24 possible keys
There are 12 distinct pitches in Western music (the 12 notes of the chromatic scale). Each one can serve as the root of a major or minor scale, giving you 24 total keys: 12 major and 12 minor. Major keys tend to sound bright or uplifting; minor keys tend to sound darker or more melancholic. That's a generalization with plenty of exceptions, but it's a useful starting point for understanding why two tracks might clash even if their BPMs are identical.
How DJs use key: harmonic mixing
When you mix two tracks together, their keys interact. If they're compatible, the transition sounds smooth — almost musical, like the second track was always supposed to follow the first. If they're not compatible, you get a clashing, off-key feeling that no amount of EQ will fix.
Harmonic mixing is the practice of choosing tracks whose keys are compatible so transitions sound melodic rather than jarring. Compatible keys are ones that share enough notes to coexist without producing obvious dissonance.
The Camelot Wheel
Music theory has a tool called the circle of fifths that maps out which keys are harmonically related. DJs have their own version: the Camelot Wheel. It remaps every key to a number (1–12) and a letter (A for minor, B for major), so compatible keys are visually adjacent on the wheel. A minor = 8A. C major = 8B. Moving one step clockwise, counterclockwise, or switching between A and B at the same number all produce compatible transitions.
The Camelot system exists specifically because "go up a perfect fifth" is harder to act on in the booth than "go from 8A to 9A." Same music theory, friendlier interface.
Key, tempo, and the pitch shift problem
Changing a track's playback speed also changes its pitch — slow it down and it drops; speed it up and it rises. Master tempo (also called key lock) corrects for this digitally, allowing you to adjust BPM without shifting the key. But at extreme tempo changes (more than 5–6% from the original), the pitch correction algorithm starts to introduce artifacts, and the key can drift slightly. Worth knowing when you're trying to harmonic-mix a track you've nudged aggressively to match a different BPM.
Auto-detection software
Software like Mixed in Key analyzes tracks and writes their Camelot codes directly into the ID3 tags of the file. When you load those tracks into rekordbox or Serato, the key is already there. No ear training required — though understanding what the numbers mean helps you make better decisions when the algorithm gets it wrong (and it occasionally does, especially on tracks with ambiguous or shifting tonality).