The I chord
Roman numerals in music refer to chords based on scale steps. The I chord can mean different things depending on the actual musical key. In the key of C major, the I chord is C, in the key of D major, the I chord is D and so on.
Chords and intervals
The table shows how scale steps and chords are related in the key of C:
| Scale steps | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Roman numerals | I | ii | ii | IV | V | vi | vii |
| Chords | C | Dm | Em | F | G | Am | Bdim |
The first scale step is C. The first note of a scale is also referred to as the tonic. Depending on the key, the I (tonic) becomes different notes. Note that small letters are used for minor and diminished.
The I chord in all keys
As already mentioned, the I chord is identical with the name of the key, so the following overview is almost superfluous.
C major key = C major
C# major key = C# major
D major key = D major
D# major key = D# major
E major key = E major
F major key = F major
F# major key = F# major
G major key = G major
G# major key = G# major
A major key = A major
A# major key = A# major
B major key = B major
So, for example, in the key of G# major, the I chord will be G#.
When relating to other chord types, a number or a sign is added to the I character. For example, Imaj7. In the key of C, Imaj7 will obviously be Cmaj7.
The i chord in all keys
When the Roman numerals are written in small characters, they represents minor. The I and the i chords are therefore not classified as identical.
A minor key = Am
A# minor key = A#m
Bm minor key = Bm
Cm minor key = Cm
C#m minor key = C#m
Dm minor key = Dm
D#m minor key = D#m
Em minor key = Em
Fm minor key = Fm
F#m minor key = F#m
G minor key = Gm
G# minor key = G#m
So, for example, in the key of E minor, the i chord will be Em.
The reason behind it all
Why not call a chord with its normal name in all situation and why bother with Roman numerals, you may think? The benefit with Roman numerals is that the I, II, III chord etcetera can be transposed to other keys. The I - IV - V progression, for example, can involve several different chords depending on the key. Using Roman numerals is a simple way to refer to a specific progression regardless of the key.
