Key detection
Letting an algorithm work out which key a recording is in, mostly so DJs can mix tracks that sit well together.
The core method comes from music psychology. Carol Krumhansl developed key profiles in the early 1980s — how well each pitch class is perceived to fit major and minor keys — and the Krumhansl–Schmuckler algorithm simply correlates a piece's pitch distribution against all 24 profiles and picks the best match. For audio, that distribution is built from a chromagram.
From the mid-2000s it became a product: tools like Mixed In Key tagged DJ libraries with Camelot wheel codes, turning harmonic mixing into picking neighbouring numbers. The failure modes are predictable. Relative keys such as C major and A minor share every note and get confused constantly, and modal music fits none of the profiles particularly well.