Musical hierarchy refers to the layered structure of musical elements, including tonal hierarchy, where notes and chords are ranked by stability (e.g., the tonic is strongest), and metrical hierarchy, which organizes time through beats and subdivisions. Other hierarchies include grouping structure (how music is segmented), time-span reduction, and prolongational reduction, which are theoretical frameworks for analyzing the relationships between different parts of a piece. A musician’s skill can also be described in a hierarchy, from basic notes and rhythms to artistic musical expression.