Roman numerals label chords by their position in the scale. Uppercase = major. Lowercase = minor. ° = diminished.

The pattern is the same in every major key: I ii iii IV V vi vii°

Numeral	Quality	C	G	D	A	E	F	Bb	Eb	Ab
I	Major	C	G	D	A	E	F	Bb	Eb	Ab
ii	Minor	Dm	Am	Em	Bm	F#m	Gm	Cm	Fm	Bbm
iii	Minor	Em	Bm	F#m	C#m	G#m	Am	Dm	Gm	Cm
IV	Major	F	C	G	D	A	Bb	Eb	Ab	Db
V	Major	G	D	A	E	B	C	F	Bb	Eb
vi	Minor	Am	Em	Bm	F#m	C#m	Dm	Gm	Cm	Fm
vii°	Dim	Bdim	F#dim	C#dim	G#dim	D#dim	Edim	Adim	Ddim	Gdim

The natural minor pattern: i ii° III iv V VI VII

Numeral	Quality	Am	Em	Dm	Gm	Cm	Fm
i	Minor	Am	Em	Dm	Gm	Cm	Fm
ii°	Dim	Bdim	F#dim	Edim	Adim	Ddim	Gdim
III	Major	C	G	F	Bb	Eb	Ab
iv	Minor	Dm	Am	Gm	Cm	Fm	Bbm
V	Major	E	B	A	D	G	C
VI	Major	F	C	Bb	Eb	Ab	Db
VII	Major	G	D	C	F	Bb	Eb

1. Identify the key. What's the tonic?
2. Find the chord's root on the scale. Count up from the tonic: 1st note = I, 2nd = ii, 3rd = iii, etc.
3. Check the quality. Does it match the expected pattern? Major chords get uppercase, minor get lowercase.

If you see a chord that doesn't fit the diatonic pattern, it's probably borrowed or secondary. That's more advanced territory, but the process is the same: find the root's scale degree, label it.

These come up constantly. Learn to spot them:

Numerals	Name	In C	In G
I - V - vi - IV	The "four chord" progression	C G Am F	G D Em C
I - IV - V - I	The classic	C F G C	G C D G
vi - IV - I - V	Axis rotation	Am F C G	Em C G D
ii - V - I	Jazz turnaround	Dm G C	Am D G
I - vi - IV - V	'50s progression	C Am F G	G Em C D

• I and V are the two most important chords in any key. Home and away.
• IV is the second most common chord after I and V.
• vi is the relative minor. Same notes as the I chord's key, minor flavor.
• ii almost always leads to V. The classic setup chord.
• vii° is rare on its own. Same function as V, just less common.
• iii is the rarest diatonic chord in pop and rock.