Jazz improvisation is a key aspect of jazz. Basically, improvisation is composing on the spot, in which a singer or instrumentalist invents solomelodies over top of a chord progression played by rhythm section instruments (piano, double bass, drum kit, etc.). While blues and other genres also use improvisation, jazz improvisation is distinguished from other genres use of this approach by the high level of chordal complexity, often with one or more chord changes per bar, altered chords, unusual chords (e.g., augmented chords), and ii-V-I progressions, all of which typically move through multiple keys within a single song. However, since the release of Kind of Blue by Miles Davis, jazz improvisation has also come to be associated with modal harmony and improvisation over static key centers, while the emergence of 1950s-era free jazz has opened up a much wider variety of styles of jazz improvisation, such as "free blowing", in which the soloists ignore the chord changes.