You may have run across some sheet music or transcriptions that use Roman numerals instead of chord names. Even if you haven’t yet, you’ll be sure to encounter them when you become a rich and famous studio musician in Atlanta, L.A. or Nashville! (Or any studio gig!)
You know that every song is in a certain key such as C or G. The key gives us the “tonal center” of a song, which is the chord to which the song naturally seems to resolve. Most songs in the key of C, for instance, will start and end with a C chord (there are exceptions of course). This tonal center, C, is known as the tonic (or root) chord, and is labeled with the Roman numeral I. The second step chord, D, is II; the third step, E, is III; the fourth, F, is IV, and so on.
Basically, you’re just moving to a higher key alphabetically. However, in a chord progression built upon a harmonized scale, not every chord will be major. Therefore we use lower case numerals (i, ii, iii, iv….) to designate minor chords. No need to get frustrated if this seems too complicated. Just accept the fact for now that because of the way scales are made – and notes within the scale are harmonized – every key has both major and minor chords.
This system allows the musician to establish the best key for, say, a vocalist. The chords keep the same intervals relative to the tonic (an interval can be thought of as the “distance” between two notes; i.e., a C to a C# is an interval of one half step, whereas a C to a D is one whole step).
Confusing? Say I’m in the key of C, and my first two chords are C and Dm. The singer says, “That’s just a bit too low for me to sing comfortably. Could we raise the key a bit?” Since my “I” chord is C, and my “ii” chord is Dm, I can mentally move up one step (that’s the interval we just discussed) . . . now the “I” chord becomes D, and the “ii” chord becomes Em.
The progression I / vi / ii / V7, in the key of C, would give the chords C / Am / Dm / G7. What would the chords be in the key of G? (G / Em / Am / D7). What about the keys of F or A?