It expresses a hierarchical segmentation of a piece into motives, phrases, periods, and still larger sections.
Metrical structure expresses the intuition that the events of a piece are related to a regular alternation of strong and weak beats at a number of hierarchical levels.
They establish tree structure-style hierarchical organizations uniting time-spans at all temporal levels of a work.
[5] The TSR analysis begins at the smallest levels, where metrical structure marks off the music into beats of equal length (or more precisely into attack points separated by uniform time-spans[6]) and moves through all larger levels where grouping structure divides the music into motives, phrases, periods, theme groups, and still greater divisions.
Prolongational reduction (PR) provides our "psychological" awareness of tensing and relaxing patterns in a given piece with precise structural terms.
In prolongational reduction, hierarchy is concerned with relative stability expressed in terms of continuity and progression, the movement toward tension or relaxation, and the degree of closure or non-closure.