Among the characteristics of a composition that can distinguish one artist's version from another is the choice of tonic. Awareness of this characteristic can itself come as a "shocking revelation" to beginners who assume that all music is composed in the key of the first scale they learn or that pieces are played only in the key of the first printed version they encounter. John McNeil went into a panic during an early jam session in which saxophonist John Handy "called the tunes in different keys." Afterwards, McNeil says, he "hid from other musicians for months," until he had made up his deficiency by relearning his repertory "in all twelve keys."

To meet the challenges of key transposition, students must train themselves to hear a piece's intervals, that is, to imagine their precise sounds, at differing pitch levels....

Beyond its variable key, a piece's precise melodic features can differ from version to version. Within an arrangement, singers or instrumentalists who carry the melody can transform it to varying degrees, engaging in compositional activities of increasing "levels of intensity" that Lee Konitz distinguishes along a continuum from interpretation to improvisation. Success at one level provides the conceptual grounding and "license" musicians need to graduate to successive levels, each increasing its demands upon imagination and concentration.

[Berliner 66]

Another variation? on a Post-Theme?