Key

A track's key is the note it is built around and the mode it is built in: A minor, G major, F sharp minor. Two tracks in keys that do not get along produce clashing notes when they overlap — and you hear it even when the BPM lines up perfectly.

It is written two ways. In musical notation, which is what a musician uses, and in a short code meant for the booth: Camelot (8A, 9B) or Open Key (1m, 2d). All three say the same thing; the last two exist so you can compare them at a glance with your hands busy.

The practical problem is not understanding key, it is having it. Key does not come with the file: some program writes it after analysing the audio, and detecting it well is a genuinely hard problem. Different programs disagree about the same track, especially in music with little harmonic content. If someone promises you perfect detection, be suspicious.

When half your set carries no key in its tags, EnergyCurve does not guess: it orders by what it can read and tells you what went unchecked. That is better than a compatibility verdict built on data that is not there.