Just What Is a ii-V-I ?
At its simplest, a ii-V-I is a three chord progression. It moves from a chord built on the second note of a major scale, to a chord built on the fifth note of the scale, to a chord based on the root. This is an extraordinarily common sequence of chords that shows up in almost every song, multiple times.
Want more detail? OK.
What is a major scale? Start by recognizing some nomenclature. Moving one fret on the guitar neck is called a ‘half-step’ (H). Moving two frets is called a ‘whole-step’ (W). Major scales are a sequence of seven notes plus a repeat of the first note an octave higher than the first. (Note: ‘octave’ or ‘oct’ = 8.) They are constructed as:
W W H - W - W W H
You will hear people describe this as two groups of four notes separated by a whole-step.
And you may have heard major scales sung as do-re-me-fa-so-la-ti-do.
Begin this sequence on any note and you produce a major scale named for the note you started on. That is, if you start the WWHWWWH sequence on a C, you produce a C major scale. Start on F# and you produce an F# major scale. Ditto any note you choose.
Sometimes it’s handy to refer to the notes in a scale by their position. In the C major scale the C is 1, D is 2, and so forth. If you happen to be playing in the G major scale, then G is 1, A is 2, … you get the idea.
We can build chords on each of the major scale notes. Let’s use the C major scale as an example.
C maj 7 C E G B
D min 7 D F A C
E min 7 E G B D
F maj 7 F A C E
G 7 G B D F
A min 7 A C E G
B dim 7 B D F A
It turns out that the chords built on the 1st, 4th, and 5th note of any major scale are always major chords. (The difference between major 7ths and dominant 7ths is for another discussion.) And the chords built on the 2, 3, and 6 of the scale are always minor chords. The chord on the 7 note is a diminished.
As a shorthand way of referring to chords, major-based chords are written with capital Roman numerals, minor-based chords are written with small Roman numerals, and the diminshed uses small Roman numerals with a superscript circle (which I can’t write using this blog text editor!) So, the chords in ANY major scale are written:
I ii iii IV V vi viio
And that means that a ii-V-I is a sequence of chords that move from the ii chord to the V chord to the I chord within ANY major scale.
