Back in the day, music was simple to understand. Your grandparents simply had "music," which usually referred to Brylcreemed crooners singing the sort of song that brought to mind cardigans, hot chocolate and a crackling fire.
After Bill Haley there was "music" and there was "rock 'n roll."
Bob Dylan popped up and we had to add "folk."
Someone paid attention to what was going on in the US and reluctantly, we opened a new file called "country and western."
From there, it went downhill. Acid rock, folk-rock, glitter rock, prog rock, disco, punk, new wave, ambient, house, trance, garage, you get the picture.
Proper music nerds like nothing more than to spend hours categorising bands. And it's got to the stage now where pretty much every band has its own sub-sub-sub-sub genre. Just to prove that they aren't like anyone else you've ever heard.
And of course you can't have any two genres actually merge - it has to be a 50:50 joint-venture, with an option to withdraw on grounds of music and aesthetic incompatibility.
Where is all this taking us tonight?
To the site of a deep-space impact, to be precise. The kind of impact where a solid, meaty planet busy on its own orbit meets a feisty asteroid that just won't take no for an answer.
Imagine you're in The Byrds, a solid, respected band touted for a while as the US' answer to The Beatles. You're all fantastic musicians, you sing glorious harmonies and you've managed to drag folk influences into the electronic rock age more convincingly than Dylan ever did.
You've had some seriously big hits: "Mr Tambourine Man", "Turn! Turn! Turn!", "Eight Miles High", "My Back Pages."
Unfortunately, your band is falling down around your ears. Either you don't get on with them, or they don't get on with you, or they have better things to do, and eventually you're left as just Chris Hillman and Roger McGuinn. What to do?
Well, what they *did* was draft in Gram Parsons.
Gram was a fairly low-profile guy at this time: no track record to speak of, but it didn't stop him from shoving The Byrds off the folk-rock highway and onto a new, dusty, unimproved road - country-rock.
"Sweetheart of the Rodeo" was the only Byrds album with Parsons on board, but it's a million miles away from the trippy "Eight Miles High," or the chiming "Mr Tambourine Man."
Where before there were 12-string Rickenbacker guitars, here were country twangs. Where before was the nervous, jittery rhythm of "Eight Miles High", here was a laid-back cross-country groove.
Today, this sound is so all-pervasive that we take it for granted. Sheryl Crow, the Eagles, Neil Young wouldn't have been able to do half the things they did without the car-crash that created country-rock. Pretty much the entire mid-80s "Americana" movement - Jason & the Scorchers, the Rainmakers, the Jayhawks, Long Ryders - owe their existence to this album and to the new genre it created.
And that's one for the nerds.