Seems an appropriate time of year for this thought. The hardy standard Autumn Leaves…
Harmonically, it’s a very simple tune. It’s basically just turnarounds in a related major and minor key pairing.
So why is there usually either an unusually lengthy discussion before it’s played or a cock up? It’s all about the key, and it’s played in many different keys. For such a simple tune, there are actually a lot of ways to go wrong.
It starts with a II-V-I in the major key, let’s say for instance Bb. So some people will call “Autumn Leaves in Bb”.
But the first chord of II-V-I in Bb is Cm7, so some people will say “Autumn Leaves, C minor”.
Actually, both of these are wrong. It also uses II-V-Is in the relative minor key, let’s say Gm, and crucially, it ends on Gm – it resolves there. So it should actually be “Autumn Leaves in G minor”. But some people don’t think like that.
You have to be very clear, especially at a jam session, otherwise it’s possible you’ll wind up with a band starting in anything up to three different keys. After this unintended comic moment, they’ll usually hear something’s wrong and adjust, but even more hilarity can ensue when people adjust independently to each other and just wind up with the same wrongness in different permutations.
I’ve found the most foolproof way to avoid this amusing, chaotic tumble of musical foliage is to say “Autumn Leaves in Gm, so the first chord is Cm7”.