Improvisation
Spontaneous creation of music while playing.
What it means
Improvisation designates the inventing of music at the moment of playing. The performer is at the same time the composer: they decide on notes, rhythms, and course of form while playing, instead of reproducing a fixed score.
Improvisation seldom happens out of nothing. It rests almost always on a frame — a chord progression, a theme, a dance model, a key — and on a stock of turns that the improviser has mastered. Freedom arises from ability, not from its absence.
It is to be distinguished from interpretation, which interprets an existing text, and from sight-reading, which reads an unknown text directly. Extempore is used with the same meaning.
In the notation
Improvised music is by its nature not notated. The score at most gives the frame: chord symbols in jazz, a bass part with figures in figured bass, a fermata over a chord as a place for a cadenza, a theme over which to vary.
Musical use
For centuries improvising was a matter of course in the musician's profession. Organists improvised in worship, continuo players their texture, soloists their cadenzas and ornaments. In the nineteenth century this practice recedes in art music, while it becomes the centre in jazz. In church music, folk music, and historically informed performance practice it is alive to this day.
In European art music, improvisation was part of the craft into the nineteenth century: figured-bass players improvised the harmonies, soloists their ornaments and cadenzas, organists whole fugues. In the course of the nineteenth century this practice declined as the score was fixed ever more exactly. It remained alive in jazz, folk music, church music, and in non-European traditions.
Performance practice
Begin with a narrow frame — few notes, a chord progression, a rhythm — and extend it step by step. Build up a stock of turns and practise them in different keys. Think in phrases and plan goal points instead of stringing note to note. And keep the pulse: an even course carries even simple ideas, a halting one spoils even good ones.