A jazz song is both a unique performance and the standard which is the abstract form of that same song. The real book of jazz standards captures the implicitly shared knowledge between experienced jazz musicians by laying out the chord progressions, bar schemes, melodies, and standard keys for a collection of songs that we could call the jazz canon. In practice, a jazz standard serves as a minimal structure to achieve maximum flexibility in performance. Because jazz musicians share the knowledge of these songs, musicians of comparable skill can start playing music together by only referring to the standard. There may be some explicit exchange of knowledge before a start if they want to change something to the performance like the key in which to play the song, the musical style of the song, or the usual order of turns taken by the soloists. After negotiating this minimal structure, they can start. 
The very fact that the structure is minimal creates the flexibility for improvisation. It is minimal because the desire to improvise is the approach to the structure. It is minimal because it is there to serve the in-the-moment uniqueness of the human beings working with the structure. In terms of the reversibility between structure and improvisation, the conscious force, or will, to improvise ensures an evolutionary tilt ‘onwards’ in the implicative force of the unique instantiation of the standard. Paradoxically, the minimal structure must also sustain the systemic identity of the song over time. However unique the improvisations in one instance of Summertime, and however far-removed the chosen style in which it is performed from its original, the song is still clearly identified as Summertime. In this perspective on the pattern of the song we find a similar dynamic as that between the song and its context.
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