Roman numerals

Roman numerals are often used for chords instead of their alphabetical name. This helps you think of their position relative to the key.

In a major key, the three major chords are:

I   - tonic
IV  - subdominant
V   - dominant

Minor chords are usually lower case:

ii  - supertonic
iii - mediant
vi  - submediant (relative minor)

So, for example, in C major you may have a chord progression:

C  Am  F   G  C
I  vi  IV  V  I

Read more: Roman numerals