A performance can either be planned out and rehearsed (practiced)—which is the norm in classical music, with jazz big bands and many popular music styles–or improvised over a chord progression (a sequence of chords), which is the norm in small jazz and blues groups. Rehearsals of orchestras, concert bands and choirs are led by a conductor. Rock, blues and jazz bands are usually led by the bandleader. A rehearsal is a structured repetition of a song or piece by the performers until it can be sung or played correctly and, if it is a song or piece for more than one musician, until the parts are together from a rhythmic and tuning perspective. Improvisation is the creation of a musical idea–a melody or other musical line–created on the spot, often based on scales or pre-existing melodic riffs.