Jazz improvisation is the spontaneous invention of melodic solo lines or accompaniment parts in a performance of jazz music. It is one of the defining elements of jazz. Improvisation is composing on the spot, when a singer or instrumentalist invents melodies and lines over a chord progression played by rhythm section instruments (piano, guitar, double bass) and accompanied by drums. Although blues, rock, and other genres use improvisation, it is done over relatively simple chord progressions which often remain in one key (or closely related keys using the circle of fifths, such as a song in C Major modulating to G Major). Jazz improvisation is distinguished from this approach by chordal complexity, often with one or more chord changes per bar, altered chords, extended chords, tritone substitution, unusual chords (e.g., augmented chords), and extensive use of ii–V–I progression, 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 come to include modal harmony and improvisation over static key centers, while the emergence of free jazz has led to a variety of types of improvisation, such as "free blowing", in which soloists improvise freely and ignore the chord changes. Types Jazz improvisation can be divided into soloing and accompaniment. Soloing When soloing, a performer (instrumentalist or singer) creates a new melodic line to fit a song's chord progression. During a solo, the performer who is playing the solo is the main focus of the audience's attention. The other members of the group usually accompany the solo, except for some drum solos or bass solos in which the entire band may stop while the drummer or bassist performs. When a singer improvises a new melody over chord changes, it is called scat singing. When singers are scat-singing, they typically use made-up syllables ("doo-bie-doo-ba"), rather than use the lyrics of the song. Soloing is often associated with instrumental or vocal virtuosity; while many artists do use advanced techniques in their solos, this is not always done. For example, some 1940s and 1950s-era bass solos consist of the bassist playing a walking bassline. There are a number of approaches to improvising jazz solos. During the swing era, performers improvised solos by ear by using riffs and variations on the tune's melody. During the bebop era in the 1940s, jazz composers began writing more complex chord progressions. Saxophone player