Harmonizing the Scale and Roman Numerals

This is where theory becomes songwriting. After this module you should never again wonder “what chord comes next?”. You will have a map.

The idea

Take a major scale. On each note, stack two more scale notes a 3rd apart. Use only notes of the scale. You get seven diatonic chords: the chord family of the key.

Degree	I	ii	iii	IV	V	vi	vii°
In C major	C	Dm	Em	F	G	Am	Bdim
Quality	major	minor	minor	major	major	minor	diminished
With a 7th	Cmaj7	Dm7	Em7	Fmaj7	G7	Am7	Bm7♭5

The pattern is the same in every major key: major, minor, minor, major, major, minor, diminished.

Change the key and watch the chords change while the numerals stay:

Roman numerals are the transposable language of songwriting. Upper case means major, lower case means minor, and ° means diminished. “I – V – vi – IV” describes thousands of songs at once. “C – G – Am – F” describes them in one key only. Think in numerals, and translate to your key or capo position last.

Notice that V is the only chord that becomes a dominant 7th when you add a 7th. That is why G7 points so clearly to C: no other chord in the key has that sound.

On your instrument

The keys guitarists use most. Play each row as open or barre chords, saying the numeral as you play.

Key	I	ii	iii	IV	V	vi	vii°
C	C	Dm	Em	F	G	Am	Bdim
G	G	Am	Bm	C	D	Em	F♯dim
D	D	Em	F♯m	G	A	Bm	C♯dim
A	A	Bm	C♯m	D	E	F♯m	G♯dim
E	E	F♯m	G♯m	A	B	C♯m	D♯dim
F	F	Gm	Am	B♭	C	Dm	Edim

A neck shortcut. With the I chord as a barre chord rooted on string 6, the IV is at the same fret rooted on string 5, and the V is two frets higher on string 5. The vi is three frets below the I on string 6. This map is the same at every fret, so you can play I, IV, V and vi in any key without thinking about note names.

Piano. Play the seven triads of C major up the keyboard using only white keys, keeping the same hand shape. You will hear the major, minor and diminished qualities appear without doing anything. The scale decides them.

Worked example

Converting a song to numerals. The verse of “Let It Be” is C – G – Am – F.

1. The key is C major, so C is I.
2. G is the 5th note of the scale and the chord is major: V.
3. Am is built on the 6th note and is minor: vi.
4. F is built on the 4th note and is major: IV.

The verse is I – V – vi – IV. To play it in G, read the G row of the table: G – D – Em – C. To play it in A: A – E – F♯m – D. You have just transposed a song without moving a single note by hand.

Exercises

1. Write the diatonic triads with numerals for G, D, A, E and F major from memory. Check against the table.
2. Take five songs you know and convert their chords to Roman numerals. You will immediately see the same skeletons repeating.
3. Play I – IV – V – I in five keys using the neck shortcut, without looking up any chord names.
4. Add 7ths: play Imaj7 – vi7 – ii7 – V7 in C and in G with the shell voicings from lesson 3.2.

Common mistakes

• Writing “iii” for a major chord. Case carries the quality. III and iii are different chords.
• Naming numerals from the first chord of the song. Numerals are measured from the tonic of the key, wherever the song starts.

Check yourself