Theory & Composition
Music theory separates a sounding piece into structural questions: when events occur, which pitches are available, which pitches combine, how those combinations move, and how the result is represented.
A score is several coordinate systems superimposed.
The categories below are not a sequence. Each answers a different structural question, and real music uses all of them simultaneously.
The circle of fifths is a map of key proximity, not the definition of harmony.
Adjacent keys share most of their pitch material, so the circle makes closely related tonal regions visible. It is one useful coordinate system inside a much larger theory of rhythm, melody, harmony, form, timbre, and notation.
Moving by perfect fifths eventually visits all twelve pitch classes and returns to the starting class.
Nearby major keys differ by only one sharp or flat, so they share many notes.
Tonic, predominant, and dominant relationships describe tendencies inside a tonal context, not universal emotional laws.