While I'm working on this, I listen to music. My collection isn't complete, even for my favorite bands, so if something's missing below, it's because I haven't gotten around to buying it (or to writing about it).

The question is answered when one observes that the major progressive rock groups are British, and looks at the way that the British and American music scenes operate.

One observer of the English-speaking music community made the observation that to get noticed in Britain, one had to be completely different from everyone else, whereas in the United State one has to be completely like everyone else. Now this is a bit of an overstatement—it understates the variety of pop music in America and radically understates the amount of borrowing that British acts have done from American groups—and does nothing more than state that the mutually-conflicting goals of novelty and familiarity are given different emphasis on either side of the Big pond.

Record company decision-making has a lot to do with this; both are driven by the necessity of turning a profit. If a company perceives that a new act's material won't sell, the company won't support that act. The companies on the different sides of the ocean have different perceptions as to what will sell; consequently, British record companies have favored novelty over familiarity. Or it could be that American record company executives, like their counterparts in the auto industry, are intent on selling what they think the customer should want, and not what they actually want. Or it could be that the logistics of travelling to visit a record company exec are much less expensive in time and money in Britain than in the US. Or it may be that British record companies cultivate more acts, but do less for each, whereas the American companies put more resources into marketing their acts, but consequently support fewer of them.

The upshot is that bands that are going off the beaten path get more attention in Britain, and are more likely to get a break. Some genres seek novelty through clever lyrical arrangements ("Achy Breaky Heart"), and a lot of rock bands accomplish distinction by taking influence from jazz, R&B, or country-western. These are actually easy ways to break away from the three-chord melody. What turns out to be the toughest row to hoe—developing more complex compositions and arrangments—leads to progressive rock.

In addition to the more complicated music of progressive rock, the themes depart from the common themes of pop music. Instead of singing about personal misfortune (a typical blues motif), personal relationship issues (all pop music), personal lifestyle celebration (Rap, Hip-hop, and country-western), or the politics of the day (new wave and country-western), progressive rock is more likely to abstract from these themes or to delve into topics outside the experience of teenagers. Progressive rock is most closely associated with the concept album. Many people don't like concept albums, but I've always felt that knee-jerk disdain for concept albums is a mark of a single-digit mental age. Music critics are better known for a lack of brains than for any surplus thereof, and it was one of them who wrote the headline OH GOD, NOW IAN ANDERSON WANTS US TO THINK.

One semi-reliable test for progressive rock is this question: Is it better with headphones? Progressive rock tends to employ a vaster dynamic range than regular pop music.

Another semi-strong indicator of a progressive act is the appearance of instrumental pieces. Pop music avoids instrumental pieces, mainly because pop music is not about music. In the titles below, tracks that are instrumental, or whose lyrics are so brief that the song remains essentially instrumental, are underlined.

I have left out three acts that could be considered progressive rock, because they often departed from the themes of pop music, and were often doing new things musically as well: The Beatles, the Who, and the Rolling Stones. I've decided to group them under the British Invasion and leave it at that.

Another rock variant that is often associated with progressive rock is the genre known as heavy metal. Heavy metal differs from progressive rock in that the heavy metal sound is based on guitars (usually over-driven), drums, shouting vs. singing, maybe some keyboard work (the Hammond organ sound, usually), and little (if any) other instruments. Thematically, heavy metal is based on death, destruction, self-destruction, misery, pagan religion, and other uber-rebellious material. Heavy metal compositions are generally no more complicated than country-western, and the only demands on the performer are (a) the ability to play fast, (b) the ability to solo well (especially the lead guitarist), (c) the ability to shout for the duration of a concert if one is the lead vocalist, and (d) the willingness to participate in some rather silly marketing efforts. The state of heavy metal can be summed up by noting that the genre produced all of its first-rate bands in the first decade of its existence, and not even one more after that.

I shall henceforth cease to employ the term "prog rock." Only people whose brains are hurt by long words would need to hack syllables off of a three-syllable word.

These are the terms I give to the various periods of a rock group's existence.

The Stone Age is the early period wherein the group is still trying to find its defining style. For any decent group this never lasts more than two albums. Progressive rock bands in this stage are still sounding much like the other bands of their time, although they may be making experimental efforts in this direction or that.

The Golden Age is when the band has found its musical style, and produces albums on which most tracks are listenable if not great. A band's Golden Age is the standard to which all succeeding efforts are compared.

The Silver Age is when the band is no longer producing consistently top-notch work, or when they're in a rut. Albums have a few dull or unlistenable tracks, and a gem here and there.

The Dung Age is when the band has completely lost its touch. They have jumped the shark. A string of two or more albums that pretty much suck constitutes a Dung Age.