You famously only need
chords to play everything, or maybe it's only
three. But how true is this really?
I decided to go look and see what was common. I looked at a large
number of transcribed songs, writing down which chords were used.
This isn't perfect: some chords are more optional than others, but
it's about right. I only wrote down the triad: C7 and Cmaj7 are just
C. I used three sources:
Singing: a songbook popular at singalongs. I flipped through and
took the chords the 277 songs that seemed most familiar to me.
Collection: a tunebook popular among contra dance musicians. I
did the same flipping through, with all three volumes, and took the
chords from 89 tunes.
Wilderness String Band Tunebook, Third Edition. All 42 tunes.
Out of these 408 songs, here's how many include each chord:
This isn't a great way to look at it, though, because it is absolute
and music is (mostly) relative. We perceive a song that goes "C F G"
very similarly to one that goes "A D E". Musicians will often use
"Roman Numeral notation" to talk about chords in a purely relative
sense. If we look at "C F G" relative to "C", or "A D E" relative to "A",
we can call them "I IV V": they are each three major chords using the first,
fourth, and fifth notes of the major scale. We indicate minor chords
by using a lowercase number: "iv" would be "Am" relative to "C", or
"F#m" relative to "A".
Here's what this looks like in C, for example:
Here's the same chart as above, but with relative chords:
This is mechanically converted, and assumes every song is using the
standard major scale, which of course they aren't: see all those "i"
chords? Instead of thinking of a song with "Am C F G" as being "i
bIII bVI bVII", we should think of it as "vi I IV V", as if it is in C
(the relative major). This focuses our distribution:
Some songs are neither major nor minor, but in other modes, typically
mixolydian. These have chords like "A G D". Instead of "A G D" being
"I bVII IV", we can think of it using the corresponding major and use "V IV I":
While it's almost always I, IV, V, and vi, we have both II/ii and
III/iii, differing on whether the third is major or minor. One way to
handle this is just to drop the third from all the chords and play
So: If we choose a few chords, what fraction of songs/tunes can we play?
We can actually do slightly better than this: while above I only
adjusted for relative minor and mixolydian, we can be more aggressive
and try interpreting the song against all 12 possible scale
degrees. But it doesn't change things very much:
There are small differences between songs and contra dance tunes:
What are those stubborn ones which we still can't play, even with this
many chord options?
Most common are ones that use multiple keys/modes, especially a
switch between major and minor. For example, "Kitchen Girl" needs "A
G" for the first half of the tune (confusingly called the "A part")
and "Am G E" for the second half.
Others, like "Be Prepared", use major versions of chords for
effect: it uses "C E A D G C" ("I III VI II V I") for a turnaround,
following the circle of fifths. You can play it with the minor
versions, but it's not as punchy.
And a few, like "Gee, Officer Krupke", just use a ton of chords in
a way I don't really understand.
As for how this applies to my own instrument-making, for now I've
gone with "I ii iii IV V vi" and an option to use open chords.
All code and chords are on github.
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