In the bleak midwinter -- Jacob Collier (and Andrew McGregor)

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 And here is what Andrew McGregor has to say about this (scroll down for the lyrics):

Musical notes are a really, really, really complicated subject:

The base standard of western music, now, is A=440 Hz, and equal temperament, that is that there are 12 other tones related by powers of the 12th root of two, meaning 12 distinct tones in each octave.

That is a convenient approximation to a set of tones you can make out of the harmonic series, which was known to the ancient Greeks… except that if you actually try that, you discover as your music gets more harmonically complex that things sound pretty bad in some combinations, and musicians start wanting to correct them so they sound ‘right’ despite being wrong.

If you tune by ear with voices, or instruments that are not entirely fixed in their tuning, you end up using something called just intonation, and as you change key the frequencies you use for certain notes change slightly. That can mean that you can change key several times, change back to the key you started on, and end up at a different pitch (shifted by an interval called a comma).

Yeah, it’s complicated all right.

Around about the 16th century several people worked out that you could do what we now call equal temperament, it seems to have been simultaneously invented in China and Holland. It became standard in the 18th century in Europe.

But… lots of contemporary music uses tuning based on guitars, and they don’t play in exact equal temperament.

Not only that, lots of contemporary music is based on blues scales, which contain a note that isn’t one of the regular set.

Arab, Japanese and Indian music each use a different set of intonation schemes… except when they don’t because they’re incorporating Western instruments (or guitars)… except when they do something like just intonation around what the equal-tempered instruments or guitars are doing to make it sound right in their heads… yeah. Complicated.

So, any attempt to define the exact frequencies of musical notes is just the start of a long, complicated journey. People have written books on the subject, and there have been several published on this subject every year for at least four hundred years. It’s that complicated.

Using different intonation schemes can be astonishingly beautiful.

Check this out… there’s an impossible modulation in this arrangement:

At one point he smoothly modulates into a key a quarter tone sharp (in exact quarter-tone equal temperament)… by stepping through something like the just intonation commas on the way there. By ear, multitracking with himself.

Lyrics

In the bleak midwinter

Frosty wind made moan

Earth stood hard as iron

Water like a stone

Snow had fallen

Snow on snow on snow

In the bleak midwinter

Long, long ago

Angels and Arc Angels

May have traveled there

Cherubim and Seraphim

Thronged the air

But only his Mother

In her maiden bliss

Worshiped the beloved

With a kiss

What can I give him?

Poor as I am

If I were a shepherd

I would give a lamb

If I were a wise man

I would do my part

But what I can I give him

Give him my heart

Give him my heart