C minor
The minor key with three flats; relative minor of E-flat major.
What it means
C minor is the minor key with three flats in its key signature: B-flat, E-flat, and A-flat. Its tonic is C, and its relative major is E-flat major. On the circle of fifths it sits three steps into the flat side.
Musical use
Since Ludwig van Beethoven, C minor has been regarded as a particularly dramatic, fatefully struggling key, shaped above all by his Fifth Symphony (1808), the Piano Sonata Pathétique (1798), and the Piano Sonata Op. 111 (1822). Other composers took up this tradition as well, such as Johannes Brahms in his First Symphony (1876).
This association of character is historically and culturally shaped, not physically grounded: on an equally tempered instrument, C minor does not fundamentally differ in sound from other minor keys. C minor should be distinguished from the like-named major key, C major, with which it shares its tonic but not its key signature.