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Tonic, Subdominant, Dominant
Builds on Triad Construction as Stacked Intervals
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Every chord in a key plays one of three functional roles: tonic (rest, 'home' — built on scale degree 1), subdominant (motion away from home — degree 4), and dominant (maximum tension, pulling back to tonic — degree 5, especially when it's a dominant 7th chord).
Roman numerals label a chord by which scale degree it's built on and whether it's major or minor in that key: uppercase for major triads (I, IV, V in a major key), lowercase for minor (ii, iii, vi). The V chord's tension comes from its leading tone — scale degree 7 — sitting a half-step below the tonic, pulling the ear to resolve upward.
⚠️ Common mistake
Assuming roman numeral case tells you the chord's role directly. Case only encodes major vs. minor triad quality — role (tonic/subdominant/dominant) still depends on which scale degree the chord is built on.
📝 Summary
Chords group into tonic (rest), subdominant (departure), and dominant (tension, resolves to tonic) functions. Roman numerals label chords by scale degree and triad quality within a key.
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