The set of notes a piece of music is based around, defining its tonal home.
The key of a piece of music is the group of notes it is built from and the 'home' note that everything resolves toward. A song in the key of C major draws mainly on the notes of the C major scale and feels resolved when it returns to C. The key signature is the notation that tells you which sharps or flats belong to that key. Understanding key matters practically: it tells you which chords and notes will sound 'right' together, helps singers find a comfortable range, and lets DJs and producers mix tracks harmonically so they do not clash — a practice often called harmonic mixing or mixing in key.
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