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Through the history of music, particularly in the so-called western world, not much has changed with regard to how chord progressions work. In most popular music genres, the kinds of progressions used are very similar to, or at least related to, the progressions used by composers for the past several centuries. In most music theory courses, chords are referred to by using Roman numerals. This way of identifying and describing chords can work quite well in the popular music world as well. Let’s take a look at how Roman numerals (which I’ll refer to as R.N.) work, and how they can be useful to songwriters.
We know that there are seven chords that naturally occur in any major key. For example, the key of C major produces the chords C, Dm, Em, F, G, Am, and Bdim.
As you can see, some of the chords are major (C, F, and G), some are minor (Dm, Em and Am), and one is diminished (Bdim). When we describe these various chords by using R.N., the common method is to use upper case R.N. for the major ones, and lower case for the minor and diminished. (Augmented chords, which occur by altering a tone in major keys, are also labeled with upper case numerals.)
It’s easy enough to apply R.N. to these various chords. Since C is the first note (also called the tonic note) of C major, a chord based on the note C gets an R.N. of I. A chord based on D in this key will produce a minor chord, so it gets an R.N. of ii (lower case, because it’s minor).
So you can represent those seven chords (C Dm Em F G Am Bdim) with these seven R.N’s: I ii iii IV V vi vii. (The vii-chord often shows a small ‘o’ to the upper right of the vii, to indicate that it is diminished.)
So what’s the use of that? It seems like you’re simply taking one naming system (C, D, E, etc), and replacing it with another (I, ii, iii, etc).
R.Ns do more than simply tell us which chords are which. They help us accomplish two things:
- They can help us transpose music to other keys, or to see relationships within chord progressions.
- They help us identify the functions of the different chords within a song.
It’s a worthwhile exercise to convert pop song progressions to Roman numerals, because it allows you to do a level of musical analysis that’s difficult to do in any other way.
Here is “Hey Jude” (McCartney & Lennon) in R.N.: I V V7 I IV I V I.
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