TonalityGUIDE - Tonal music basic theory for Undergraduates
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Hierarchy is defined in the OED as 'a system or organisation in which people of groups are ranked one above the other according to status or authority.' In a musical context it refers to the fact that some notes and chords are structurally more important than another. The embellishments described in the voice-leading section of the Toolkit, for example are more 'important' in this sense than the notes they decorate. Another example might be the tonic being more important that chord vii for defining a sense of key.