The chronological hierarchy that underlies most music.  Most typically expressible as a binary tree.  This is why western rhythmic notation uses terms related by powers of two, i.e. 1/2 notes, 1/4 notes, 1/8 notes, 1/16th notes, etc.  This hierarchy continues in the other direction as well:  music tends to be organized into measures (bars) of four beats long, and phrases of 4, 8 and 16 bars.  This is especially true of most improvised music, where such rigorous "whens" make for more flexible "whats". 

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See Also: JazzProgrammer