Similarly, musical improvisation is the act of writing a song while performing it, a technique found most often in jazz and bluegrass (but can be traced back to renowned classical improvisers like Handel and Bach). Of course, it's a little more complicated than an imaginative children's game. Though improvisation is a highly creative and flexible technique, it requires great skill on the part of the musician. A musician involved in an improvisation must have a detailed knowledge of chord structure and complicated scales and modes. The musician must also have an intuitive ability to structure a song on the fly; great improvisation thrives on its ability to sound not improvised but rather wholly composed. That illusion, the ability of a song to seem anything but spontaneously made up, is part of improvisation's allure.
There are two basic forms of improvisation: structured improvisation and free improvisation. Structured improvisation, though a contradiction in terms, is the most common of the two. In this form, musicians will use a pre-determined series of chord changes, usually held down by the rhythm section, as the song's base. The lead instruments in the improvisation (also pre-determined) then have the freedom to create new melodies and harmonies from these pre-determined chords. The flexibility of this improvisation form is dependent on the flexibility of the chord changes, and the musicians involved must be able to play exactly what they hear in their heads, as some complicated changes may not allow for large deviations.
Free improvisation, on the other hand, is far more like a game of House or Doctor -- it has no rules. Instead of focusing on harmony or melody, free improvisation focuses on the feeling and texture of the music and the way the instruments complement each other. This form tends to be far more experimental and rarely adheres to one style or genre or music -- it is, quite simply, what it is. Notable musicians who play free improvisation include Conny Bauer, Evan Parker and Hans Reichel.