Key Takeaways
- Classical harmony uses 18 core chords (the Big 18) out of 49+ theoretically possible diatonic chords in major alone
- The Big 18 grid organizes chords by Roman numeral (column) and bass scale degree (row) — fluency requires reading both dimensions
- Bass voice is as important as Roman numeral: chords sharing a bass note (e.g., ii6/5 and iv) often sound more alike than chords sharing a root (e.g., ii6/5 and ii)
The Big 18 Chords ▶ 1:48
- 49 possible chords in major (7 triads × root/1st/2nd inversion + 7 seventh chords × 4 inversions) — classical composers regularly used only 18
- Parentheses in the grid = a single square holds both a triad and its seventh-chord version (same bass note); e.g., ii(7) = ii and ii7 grouped together
- I and V7 are the only Roman numerals appearing in all inversions; III (mediant) has no inversions in the Big 18
- VI and VII appear in one inversion only; II and IV have no second-inversion chords
Big 18 in Minor ▶ 5:00
- Same grid structure; chord qualities shift (I/IV → minor, vi → major, ii chords → half-diminished)
- All Big 18 chords use raised scale degree 7 (leading tone) — the lowered subtonic (♭7̂) never appears
- Root-position ii° triad omitted in minor (and de-emphasized in major) because of its diminished quality
Grid Patterns to Memorize ▶ 6:27
| Bass degree | Chords available |
|---|---|
| 1̂ | I, ii°4/2 |
| 2̂ | ii(7), V4/3, vii°6 |
| 3̂ | I6 only |
| 4̂ | ii6(5), IV |
| 5̂ | I6/4, V(7) |
| 6̂ | IV6, vi |
| 7̂ | V6(5) only |
Bass Voice & Chord Identity ▶ 11:47
- ii6/5 shares 3 notes with both ii and iv — but shares a bass note only with iv, making it sound closer to iv in practice
- Counterpoint in outer voices (driven by the bass) shapes perceived character as much as Roman numeral labels
- Grid rows highlight these same-bass relationships; don't only think in columns (same root)
Building Fluency ▶ 9:53
- Goal: instant translation between Roman numerals + figures ↔ bass scale-degree lines (both directions)
- Practice strategy: master C major and C minor first; add keys gradually; coordinate ear, brain, and fingers at the piano
- Interactive study tool (Flash required):
sethmonahan.com/big18.html