... is by whether it puts me to sleep. And I mean that in a good way.
At the end of 2020, I got a set of statistics from Amazon Music, telling me which songs I had streamed the most over the course of the year. The winner, by far and which surprised me, was the first side of Jethro Tull's Thick as a Brick. And that's because, more often than any other song, I'd tell Alexa to start playing it when I lay me down to sleep at night. And usually, by the time the youngest of the family was ruling with authority, I'd be off to dreamland. Not because it's boring -- it isn't -- but because it's ... well, pleasant.
I've mentioned before that I don't really think of Jethro Tull as "progressive," precisely because my favorite tune of theirs is "Locomotive Breath," which just feels like hard rock, not "progressive," to me, and which will never, ever put me to sleep. But Thick as a Brick clearly is prog, no doubt about it.
Maybe the reason I don't think of Rush or Pink Floyd as "progressive" is because I just don't get somnolence out of their music. I'll probably never go to sleep listening to 2112 or Wish You Were Here. Your mileage may vary.
I'm about 100 pages into David Weigel's The Show That Never Ends: The Rise and Fall of Prog Rock (not an affiliate link) and enjoying it immensely. I'll probably give it a more thorough review later, but for now I'll just say that it may be the most enjoyable "rock music history/fandom" read I've picked up since Stephen Davis's Hammer of the Gods (not an affiliate link). Unlike the latter book, of course, Weigel's isn't about a single band, but about a fuzzily defined genre. But it's still absorbing.
Confession time: Before yesterday, I had never listened to King Crimson. I mean, really listened, in earnest. I'd heard bits here and there, and found them interesting in a way, but never bought an album or asked a streaming service to throw them at me.
Last night, under Weigel's influence, I said "Alexa, play In the Court of the Crimson King," and before the album was over I was soundly and restfully asleep. It was interesting, but also enjoyable enough that ... zzzzzzzzzzzz, with a smile on my mind. Which was probably not their intent at all, and I'd no doubt respond differently to a live performance with light show, etc., but which is, again, a good thing. I'll probably put it on again later today, when I'm not trying to sleep, and like it just as well.
So, yep, King Crimson is "progressive."