I've been wanting to get back into trying to understand music theory for a while, and have finally taken the opportunity to spend ten minutes to dig into something that's been on the back of my mind for a long time: Why is it I like the tune to Oh Come, Oh Come, Emmanuel so much?
I guess I only think about this for a small fraction of the year, with it being a Christmas song. Lyrically, it's probably a bit of a political mess right now, but I'm focussing on the melody.
The obvious answer is "Almost every Christmas carol is in a jolly major key, and this hymn is clearly something more minor", but it sounds a little funkier than simply being in a minor key. I took a look at the score.
Looked at through the lens of classical music theory, it's pretty close to a minor key - perhaps fitting into what's allowed by a melodic minor scale. The minor-ness is announced right at the start, with the initial interval going from 1 to a flattened 3. So far, so minor.
What is interesting, though, is that some way into the tune we find out that the sevenths are not raised. This is not the harmonic minor. Maybe we can squint and call it the melodic minor? (If I play the piece with raised sevenths, making it a conventional minor scale, it's much less interesting.)
However, this isn't the only way to look at it. Not everything has to be tonal major/minor. The alternative is to realise that the melody is played with the notes of the major scale, only centred on the third note of the major scale. Or, in the framework of musical modes, it's in the Aeolian mode (which I now discover is also known as the natural minor!).
So, there we go. I think I like the way it's in the Aeloian mode, and that's nice and unusual in the world of cheerful Christmas songs.