A common convention for representing chords is to use upper- and lowercase Roman numerals (e.g. I, ii, iii, IV, etc.). You might run across this convention in books, articles, and songwriting apps.
Let’s look at how this notation works and how it can be useful for a songwriter. Since I use this notation throughout the site, this post will also serve as a reference for regular readers.
If you’re rusty on your Roman numerals, here’s a quick refresher of the ones we’ll use. Luckily, we only care about 1-7:
Arabic: 1 - 2 - 3 - 4 - 5 - 6 - 7 Roman (upper): I - II - III - IV - V - VI - VII Roman (lower): i - ii - iii - iv - v - vi - vii
Scale degrees and triads
Before we can talk about representing chords by number, we need to consider “scale degrees”. A scale degree is the position of a note relative to the root note of a scale. For example, in the key of C major, we have the following notes (with Arabic numerals representing scale degrees):
C - D - E - F - G - A - B 1 - 2 - 3 - 4 - 5 - 6 - 7
We say E is the 3rd scale degree of C major, for example, because it is in the 3rd position in the scale.
Each note in a scale can form the basis for a chord in that scale. Starting from a note, you skip notes to get a set of 3. For example, the C major chord is made up of the notes C, E, and G. And the E minor chord is made up of the notes E, G, and B.
The important point is that C is major here because picking notes from the C major scale in this way starting from C forms a major chord. And E is minor here because picking notes from the C major scale in this way starting from E forms a minor chord.
Chord scales and Roman numeral notation
Every key has a distinctive sequence of chords, which make up what’s called a “chord scale”. We just saw, for example, that in the key of C major, the triad (3-note chord) starting on C is major and the triad starting on E is minor. In C major, the chord scale is as follows:
C - Dm - Em - F - G - Am - Bdim
Each of these chords results from forming a triad on a scale degree using only notes from the scale.
Roman numeral chord notation combines the idea of scale degrees with the idea of chord scales. Each symbol in this notation tells you two things: (1) the distance of that chord from the root note of the key (the scale degree) and (2) the type of chord that it is (major, minor, diminished, etc.).
Let’s look at the chords in the C major key again as an example:
C - Dm - Em - F - G - Am - Bdim I - ii - iii - IV - V - vi - vii°
Notice that the I is uppercase. Uppercase means major. The iii, on the other hand, is lowercase. Lowercase means minor. So the iii in the key of C major represents the E minor chord.
There is one chord that stands out above, and that’s the vii°. That ° symbol means diminished. Generally, we avoid working with diminished chords on this site, just because it can be very tricky to make them sound good and they’re pretty rare in popular songs. So if you’re a beginner, you can just ignore them for now.
Benefits of the Roman numeral notation
There are a number of advantages to Roman numeral notation. We’ll consider two of them:
- The notation makes transposing a progression between keys easier.
- The notation helps us recognize patterns that apply to any key.
For example, imagine we’re discussing the chord progression C-Am-G. If you wanted to transpose this to the key of E major, you’d have to figure out which scale degrees each of these chords corresponds to. Once you worked out that this was I-vi-V, then you would find the I, vi, and V chords in E major and complete the transposition. That would have been easier if you’d had the degrees upfront.
More importantly, thinking of chord progressions in terms of Roman numerals helps you learn patterns that apply to any key. For example, the V chord often pulls us toward the I. This is true whether we’re in C major (the G chord pulls us toward the C chord) or E major (the B chord pulls us toward the E chord) or any other major scale.
It’s nice to be able to represent that without talking about specific keys. That’s part of the power of this notation.
For an in-depth introduction to thinking of the chords in major keys this way, check out my Practical Chord Progressions series of posts.
Beyond the major scale
The Roman numerals we’ve been discussing are based on the major scale. They tell us the distance of a chord from the root note of the key. But if we switch to keys other than major, the distance between degrees and the root note can change.
For example, the 3rd note in C major is E, but the 3rd note in C minor is Eb. The 3rd chord in C major is E minor, but the 3rd chord in C minor is Eb major.
We write this third chord in the minor key as bIII (i.e. flat 3 major). If you don’t understand the theory behind this, it doesn’t really matter. You can just use a table to see what chords are in the key.
For example, here are the chords in C minor:
Cm - Ddim - Eb - Fm - Gm - Ab - Bb i - ii° - bIII - iv - v - bVI - bVII
What’s interesting about these chords in the minor key is that you can borrow them when writing songs in major keys. For example, rock songs in major will often borrow the bVI (Ab) or bVII (Bb). Or you can use an old trick of using the minor iv chord (Fm), possibly right after the major IV chord (F).
You can actually borrow any of these chords, and even the parallel minor i (Cm) can be used in a song you’re writing in “C major”.
In popular music the distinction between major and minor can start to blur quite a bit because of this possibility of borrowing.
A note on the Nashville system
One final note: there’s nothing special about Roman numerals.
For example, there is another approach called the Nashville system. The Nashville system uses Arabic numerals, and writes the same information in a different way:
C major: C - Dm - Em - F - G - Am - Bdim 1 - 2m - 3m - 4 - 5 - 6m - 7°
This is probably easier for a lot of people. But since the Roman numeral system is more common in books, articles, and other resources, I stick with it to help prepare you to do your own research. It’s a lot easier to read the Nashville system if you know the Roman numeral system than it is to read the Roman numeral system if you only know the Nashville system.