Talk about progressions and you get numerals instead of chord names. It exists so that the same conversation works in any key.
Names like "the four-chord loop" or "the Canon progression" do not refer to specific chords. They refer to a sequence of positions inside a key, and numerals are how those positions get written.
Number the scale degrees
Take the seven chords from last chapter in C major and number them. The numeral just says which scale note you stacked from.
- I = C (first)
- ii = Dm (second)
- iii = Em (third)
- IV = F (fourth)
- V = G (fifth)
- vi = Am (sixth)
- vii° = Bdim (seventh)
Naming a chord by its position in the key. G in the key of C is V; A in the key of D is also V. Same number, same job.
The same shape in any key
Here is I-V-vi-IV in three keys. Every chord is different; the shape you hear is the same.
- Key of C (C, G, Am, F)
C | G | Am | F
- Key of D (D, A, Bm, G)
D | A | Bm | G
- Key of F (F, C, Dm, B♭)
F | C | Dm | Bb
Changing key to suit a singer's range is routine. If you remember "I-V-vi-IV", you can rebuild the same progression in the new key immediately. Remember it as chord names and you have to relearn it key by key.
Generate over I-V-vi-IV in D. Compare it with the C version from chapter one: same shape, different height.
Generate over I-V-vi-IV in DA numeral is not a chord name; it is a position in the key. That is why moving the key does not rename the progression.
Quick review
Everything here is answerable from this chapter. Pick an option and the answer appears straight away.
Question 1
What do roman numerals (I, V, vi) name?
Question 2
What does lower case (vi and friends) tell you?