In descriptions of melodic structure, "pitch proximity" is usually defined as the tendency for small pitch intervals to outnumber large ones. This definition is valid as far as it goes; however, an alternative definition is preferable. The alternative defines pitch proximity in terms of two more basic constraints—a constraint on tessitura (or pitch distribution) and a constraint on "mobility" (or freedom of motion). This new definition offers several advantages. Whereas the usual definition predicts only interval size, the new definition predicts interval direction as well. The usual definition predicts small intervals generally, whereas the new definition predicts context- sensitive variations in interval size. Finally, if the new definition is given the first few notes in a melody, it can assign a probability to each of the pitches that could occur next. In sum, the new definition offers a more precise and detailed description of melodic structure.

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