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Music that never deviates from the established key is purely diatonic.
Dia, as in diagonal or diametric, literally means through or across. Tonic
means tones, or more specifically, key. Diatonic music, therefore, goes
through the tones (or across the key). Foreign elements to the key are
chromatic. Both diatonic and chromatic harmony comprise the field of study
known as tonal analysis.
In tonal analysis, the foreground perspective deals primarily with chords.
Middle-ground analysis explores how those chords relate to the tonic and
each other through harmonic progression and ultimately cadence.
Background analysis examines musical texture and form.