Step by step
Using Roman numeral notation to describe chord progressions
1 Let’s start off in traditional fashion by looking at the C major scale – seven notes from C to B, played on the white notes of the piano keyboard. When talking about scales such as this, we refer to the notes not just as notes, but as ‘degrees’. So we can also say that there are seven degrees in a major scale.
2 This is what it looks like when we number these degrees from 1 to 7 using Arabic numerals. The root note of C takes the number 1, D is number 2, E is number 3 and so on, up to B as number 7. But what happens when we harmonise the major scale by adding alternate notes from the scale to each degree to make three-note chords, or ‘triads’?
3 Here’s where the Roman numerals start to become useful: if we replace the Arabic numerals with the Roman equivalents, we can use upper or lower case Roman numerals to denote whether a chord is major or minor. This frees us from the constraints of the current key, allowing us to construct a formula for a chord progression that works in any key.
4 Let’s illustrate this a bit more clearly by looking at a basic progression in the key of C major. We’ve got the chords C major, F major, G major and C major. These are all chords taken from our harmonised C major scale, making it what’s known as a ‘diatonic’ progression – all the notes in all the chords are taken from the C major scale.
5 Because the chords are diatonic – they belong to the harmonised scale – we can swap their names for Roman numerals. So our tonic chord, C major, is going to be the I chord. F major is the fourth chord in the harmonised scale, so this will become the IV chord, while G major, by the same process, becomes known as the V chord, making this a I - IV - V - I progression.
6 But what if we now switch the key from C major to, say, D major? We still have the same progression, but the chords that we’re playing are going to be different because we’re now in a different key. Here’s what the harmonised D major scale looks like, with our diatonic chords labelled with Roman numerals.
7 Note how the pattern of major, minor and diminished chords is the same for D major as it is for C major. So applying the formula for our original progression, the I chord of D major is D major, the IV chord will be G major and the V chord will be A major. So in this key, our I - IV - V - I progression is played as D major, G major,
A major and D major.
8 How about something a little bit more F# complex in, say, major? Here’s a F# Bbm Ebm progression that goes - - -BAbm C# F#. - - When we match these chords F# up against the harmonised major scale Abm, Bbm, C#, Ebm, (F, B, Fdim) and label them with Roman numerals, it translates into the progression I - iii - vi - IV - ii - V - I.
9 Let’s now switch to a different key – how about A major? The diatonic C# chords for this key are A,Bm,m,D,E, F# G# m and dim. So, if we apply our Roman numeral formula for the progression (I-iii-v i-IV-ii-V- I ), we get the chords C# F# A- m-m- D-B m-E- A.
10 So far we’ve been looking at diatonic chords, made up only of notes from the scale relating to the key we’re in. But how do we label non-diatonic chords – ones that don’t belong to the key, like borrowed or modal interchange chords, for instance? This progression in the key
Ab of C major contains an major chord which doesn’t belong to the key of C major.
11 With chords like this, where the root note has been sharpened or flattened relative to the regular diatonic major scale chords, we place a flat or sharp symbol in front of the Roman numeral for that chord. So since A is the sixth chord of C major, our
Ab borrowed major chord gets labelled as bVI.
Upper case because it’s a major chord, with a flat to show it’s a flattened VI chord.
12 To round off, last month’s Easy Guide, which dealt with modal interchange, featured a chord chart of all the chords available in all of the harmonised modes based on the root note of C. This version of the chart uses Roman numerals instead, meaning that it can be applied to any key, not just the key of C.