Module 3: Theory II: seventh chords and the ii–V–ILesson 2beginner⏱ 60 min + one week of daily drills

Diatonic seventh chords and chord functions

Harmonise the major scale in seventh chords, name every chord by Roman numeral and by function, transpose the result to all twelve keys, and meet the diatonic chords of the minor key.

Why this is the map of every key

In Diatonic triads and Roman numerals you built a triad on each degree of the major scale and labelled it with a numeral. Jazz does exactly the same thing with seventh chords. The result is a set of seven chords that belong to a key, each with a fixed quality and a fixed job. Once you know that set, a chart stops being a list of unrelated symbols and becomes a story: this chord is home, this one is leaving home, this one is pulling back. Learning the diatonic sevenths is also what lets you transpose a tune to another key in your head, because you remember numerals, not letters.

Harmonising the major scale in sevenths

Stack four alternating scale notes on each degree of C major (every other note of the scale, which is the same as stacking thirds) and you get the chords below. The engine generated this table, so trust it over your memory.

Read the pattern of qualities down the table: maj7, m7, m7, maj7, 7, m7, m7b5. That pattern never changes. In any major key, the chords on degrees 1 and 4 are major sevenths, the chords on 2, 3 and 6 are minor sevenths, the chord on 5 is a dominant seventh, and the chord on 7 is half-diminished. If you can play the major scale in a key, you can name its seven chords without writing anything down.

Roman numerals for seventh chords

The numeral system from Module 2 extends naturally. Upper-case numerals mean a major 3rd, lower-case a minor 3rd, and a suffix gives the seventh:

• Imaj7 and IVmaj7: major seventh chords (some charts write IΔ7, IVΔ7).
• ii7, iii7, vi7: minor sevenths (also written ii–7 or iim7).
• V7: the dominant.
• viiø7: half-diminished (also viim7b5).

The second column of the diatonic table above shows these numerals as the engine prints them. Memorise them as a sentence: one-major, two-minor, three-minor, four-major, five-dominant, six-minor, seven-half-diminished. When you later meet a chord that is not in the key (a bII7, a V7/ii, a #iv°7) the numeral will say so, and that is how you will know to expect a different sound.

Function: what each chord does

Seven chords, three jobs. Every diatonic chord belongs to one of three functions, and the function predicts where the chord will go next.

Tonic: Imaj7, iii7, vi7

The tonic function is home. Imaj7 is the resting point of the key, and most tunes start and end on it. vi7 shares three of its four notes with Imaj7 (compare the rows in the table) and is the relative minor; it is the most common substitute for I, and a I–vi move is often the first thing that happens in a tune. iii7 also shares three notes with Imaj7 (the top three) and works as a lighter tonic, usually on its way to vi7.

Subdominant: ii7, IVmaj7

The subdominant function is "leaving home". IVmaj7 and ii7 share three notes and are interchangeable in most situations; jazz prefers ii7 because its root is a perfect 5th above V, which gives the strongest root motion into the dominant. A chord with subdominant function almost always moves to a dominant chord.

Dominant: V7, viiø7

The dominant function is tension that must resolve to the tonic. V7 contains the tritone of the key (the notes on degrees 4 and 7, which in C major are F and B; find them in the G7 row of the table). The 7th of V7 falls by a half step to the 3rd of I, and the 3rd of V7 rises by a half step to the root of I. That two-note resolution is the engine of Western harmony and of every ii–V–I you will ever play. viiø7 contains the same tritone and the same pull, but it is rare in jazz as a dominant; you will meet it far more often as the ii chord of the relative minor (lesson 4).

Playing the harmonised scale

Here is the C major scale harmonised with the drop-2 shapes from lesson 1 on strings 5–4–3–2. Every chord is in root position, so the root on the 5th string simply climbs the scale. Say the numeral as you play each chord.

\tempo 72
.
\ts 4 4
:2 (3.5 5.4 4.3 5.2){ch "Cmaj7"} (5.5 7.4 5.3 6.2){ch "Dm7"} | (7.5 9.4 7.3 8.2){ch "Em7"} (8.5 10.4 9.3 10.2){ch "Fmaj7"} |
(10.5 12.4 10.3 12.2){ch "G7"} (12.5 14.4 12.3 13.2){ch "Am7"} | (14.5 15.4 14.3 15.2){ch "Bm7b5"} (15.5 17.4 16.3 17.2){ch "Cmaj7"} |

Watch your fretting hand as you climb: the shape changes only at the chords whose quality changes. Imaj7 to ii7 changes three fingers; ii7 to iii7 changes none (same shape, two frets up); iii7 to IVmaj7 is back to the first shape; V7 is the dominant shape; vi7 is the minor shape again; viiø7 is the one half-diminished shape.

Now the same harmonised scale as arpeggios, in F major, staying inside one position (frets 1–5) so that you also hear the key change the colour of the chords.

\tempo 80
.
\ts 4 4
:4 1.6{ch "Fmaj7"} 5.6 3.5 2.4 | 3.6{ch "Gm7"} 1.5 5.5 3.4 | 5.6{ch "Am7"} 3.5 2.4 5.4 | 1.5{ch "Bbmaj7"} 5.5 3.4 2.3 |
3.5{ch "C7"} 2.4 5.4 3.3 | 5.5{ch "Dm7"} 3.4 2.3 5.3 | 2.4{ch "Em7b5"} 5.4 3.3 3.2 | :1 3.4{ch "Fmaj7"} |

Finally, a functional progression you will see in thousands of tunes: Imaj7–vi7–ii7–V7, played here in G major with three-note shell voicings (root, 7th, 3rd) so that the hand can stay in one area. Shells are the subject of Module 4; for now, just use them to hear tonic, tonic substitute, subdominant and dominant in a row.

\tempo 90
.
\ts 4 4
:1 (3.6 4.4 4.3){ch "Gmaj7"} | (7.5 7.3 8.2){ch "Em7"} | (5.6 5.4 5.3){ch "Am7"} | (5.5 5.3 7.2){ch "D7"} |

The diatonic sevenths in all twelve keys

The pattern of qualities is fixed, so once you know a key's scale you know its chords. The table below gives all seven chords in every major key. Do not try to memorise it as a table; use it to check yourself while you work through the exercises, and to look up a key you are less familiar with.

Two things to notice as you scan the table. First, the key signature does the spelling for you: a key with flats has flat roots, a key with sharps has sharp roots, and the chord qualities never change. Second, each key shares chords with its neighbours on the circle of fifths. The V7 of one key appears in no other key (one dominant per key), which is why hearing a dominant chord tells you what key you are in.

Diatonic chords in minor

The relative minor uses the same notes as its relative major, so it also uses the same seven chords, but numbered from a different tonic. Here is A minor, the relative minor of C major, harmonised from the natural minor scale:

Compare this with the C major table: identical chords, new numerals. The tonic is now i7, the ii chord is half-diminished (iiø7), and the chord on degree 5 is a minor seventh (v7), which has no tritone and therefore no pull back to the tonic. That weakness is the whole reason minor-key harmony is more complicated than major: to get a proper dominant, composers raise the 7th degree of the scale, which produces the harmonic minor scale and a V7 chord. Lesson 4 is devoted to that; for now, know that a chart in a minor key will usually show V7 (not v7) and will usually show the tonic as a m6, mMaj7 or m7 chord depending on the style.

Exercises

1. Say the chords of a key. Pick a key, play its major scale once, then name the seven diatonic sevenths aloud in order without playing. Check against the progression table. Do four keys a day, moving around the circle of fifths.
2. Harmonised scale in drop 2. Play the C major example above, then move the whole thing to F, Bb and G (the root of chord I on the 5th string at frets 8, 1 and 10). Say the numerals.
3. Harmonised scale in drop 3. Do the same with the drop-3 shapes from lesson 1 on strings 6–4–3–2, roots on the 6th string.
4. Function drill. Play random diatonic chords from one key and say their function (tonic, subdominant, dominant) before each one. Then play I–vi–ii–V, I–ii–iii–IV and IV–V–iii–vi and hear how the functions flow.
5. Arpeggios in position. Play the F major arpeggio example, then transpose it to Bb major (everything up five frets) and G major (everything up two frets, or down three).

Quiz