Creation of music in real time. Improvisation usually involves some preparation beforehand, particularly when there is more than one performer. Despite the central place of notated music in the Western tradition, improvisation has often played a role, from the earliest organum through the use of continuo (partially improvised accompaniment played on a bass line) in the 17th and 18th centuries. It has taken forms such as creation of a melody over a bass line for dancing, elaborate ornamentation added to a repeated section in an aria, keyboard variations on popular songs, concerto cadenzas, and free solo fantasias. Perhaps at its lowest ebb in the 19th century, improvisation returned to concert music in experimental compositions and in authentic performances of older music. Its most important contemporary Western form is jazz. It is also a defining feature of the raga.