My copy of Rumours cost me a pound from a record fair stall bargain bin. I know this because the sticker’s still on the sleeve. I don’t know how many other cheap copies I had flicked past over the years before choosing on a whim to buy that one. Dozens, I would guess. Rumours was everywhere, and nobody seemed to want it.

Rumours is still everywhere. And everybody seems to want it. Here in Brighton, the Komedia venue is putting on a Rumours club night this month. The 500-plus capacity sold out so quickly they’ve added a second date for it. Soon afterwards, Fleetingwood Mac, “a young and exciting new tribute to the legendary band Fleetwood Mac”, are scheduled to perform down the road at Sticky Mike’s Frog Bar. That a tribute act to a band that formed 50 years ago should bill itself as “young, exciting and new” tells us something of our current pop culture. But what’s most significant is that the audience will almost certainly be young, and excited - just as the clubbers are whom I’ve seen going bananas whenever (as often happens lately) ‘Go Your Own Way’ or ‘The Chain’ are played out. They’d rather jack to Fleetwood Mac.

How did this happen? Is it simple retromania that makes a 40-year-old album - considered emblematic of AOR first time around - so much of our moment, adored by people half its age?

The obvious answer is, Rumours is a classic. Which it is. But not one of those classics whose reputation has held at a constant high. Go back ten years and you might have drawn a few nostalgic 40-somethings to a Rumours club night. 20, and I doubt you’d have been able to give away tickets. 30, when the singles from Tango In The Night were in the charts, and The Reynolds Girls two years away from the Top Ten, and the whole idea would have been weird. I had a line, then, that Fleetwood Mac were “the world’s best boring band.” I was very young and I thought that was clever. Maybe it was, but it wasn’t right. Either I didn’t know how to listen, or more likely, I didn’t yet know how to be true to what I heard. Because had I been honest, I would have acknowledged that I had never once been bored by Fleetwood Mac (of whose work I had heard only Rumours, inevitably, and Tango, which was also, like its hit single, everywhere.)

Now I think I know why. It’s not just the timeless emotional impact of the songs, which speak to anybody who has had any dealings whatsoever with love. It’s that, arriving at the same moment (according to the Established Hindsight Narrative, without whose benefit events are obliged to occur) as the idea which was supposed to kill things like it off, Rumours turned out to be not at all the kind of thing that was supposed to be killed off. It was as discreetly radical as Punk Rock and New Wave were overtly so. It has endured because it was, is, always will be, young, exciting, new. There isn’t another album quite like it.

Except there is. There’s another album very much like it. It’s Tusk, the Fleetwood Mac album that followed it, and which both the Established Hindsight Narrative, and its creators’ intent at the time, would portray as a dramatic swerve away from it. The narrative says that Rumours was the safe bet that duly paid off, Tusk the mad, brilliant, risky experiment that went commercially awry. But Rumours was nothing like a safe bet, and only its success has made it seem so. It was a wild gamble that wildly succeeded. Had the albums been made the other way around, perhaps that narrative would make more sense. Tusk might have sold a tenth of what it eventually did, and Rumours would then seem tempered for the market. But emerging as it did, Rumours created its own market, and it is to the band’s credit that they chose not to replicate it intentionally for that market.

I say, the band, but really, we’re talking about Lindsey Buckingham here. It is no disservice to Fleetwood Mac’s other creative engines, to the thrilling luminescence of Stevie Nicks and the melodic class of Christine McVie, to allow that Buckingham, perhaps as much through sheer egotism and monomania as anything else, welded together the band’s chaotic elements and propelled it through its glory era as if piloting a soapbox derby car. If he hadn’t, maybe one of the others would have, and maybe it would have resulted in something greatly different and differently great. But he was the one who did it, and on Tusk he did it with furious avant-garde purpose. As we are reminded every day, it is the most determinedly and ruthlessly crazy people who get to define reality. In Fleetwood Mac that person was Buckingham; fortunately, he was a bona fide visionary. I watched Buckingham play ‘Tusk’, the song, with Fleetwood Mac in London not so long ago. It was my favourite part of the show. I’m not sure I’ve ever seen a man look so vindicated. He knew he wasn’t wrong - then, and now.

The stories told about Buckingham’s work on Rumours and Tusk are relayed with a head-shake and a chuckle, and as if they are indicative of frenzied cokehead megalomania. They probably are. But if they were told about somebody whose primary reputation is as an experimental artist, they would be understood as evidence of this. All that stuff about lying on the floor to record the vocals, about playing percussion on Kleenex boxes and legs of lamb . . . if, say, Tom Waits, or David Bowie, had done that, nobody’s first reaction would be to snicker about what a drug-addled loon he was, in thrall to the excesses of Seventies stardom. They’d marvel at his willingness to try whatever his sonic explorations demanded. This is the thing to remember about Buckingham: maybe he was a drug-addled loon in thrall to the excesses of Seventies stardom, but that’s not the point. The point is that he did this stuff, and it worked. Even when it didn’t work in the way he or the band might have intended. Especially when it didn’t.

You didn’t see the Eagles or Foreigner or Toto making records that sounded this way, or more crucially, felt this way. The essence of AOR lay in its practitioners’ command of their craft, of their technique: they were always perfectly in control of what they did. Not Fleetwood Mac. It’s not that they were inept; far from it. It’s that, driven by Buckingham (himself technically gifted to a remarkable degree), they refused to be confined within the limits of their aptitude. They were impulsive where their peers were meticulous. Even on Rumours, they were frequently all over the shop. ‘Go Your Own Way’ sounds as it does - like a runaway HGV gathering pace as it rolls diagonally downhill on unbalanced and mismatched wheels- because Buckingham wanted Mick Fleetwood to copy Charlie Watts’ drumming on ‘Street Fighting Man’. But Fleetwood, as instinctive a drummer as ever raised a stick, hadn’t a prayer of doing that. He got it wonderfully wrong, and getting things wonderfully wrong is the key to moving pop forward - the mutation in the gene that allows it to evolve.

Toto’s Jeff Porcaro, a clinical, masterful technician, would watch Fleetwood play the song live, fascinated, trying to work out how Fleetwood did it. He couldn’t, because Fleetwood himself had no idea how he did it. That was where its splendour lay. As post-punks and New Wavers consciously sought new ways to accomplish what technical limitations would previously have forbidden them, Fleetwood Mac did the same thing inadvertently. ‘The Chain’ is a quite deranged track, a Frankenstein’s monster of a thing, stitched together from parts of other things, lumbering around, smashing everything up. It is magnificent: a sustained, ragged, ever-unfolding howl of jilted grievance to rival any break-up song in pop. And this, as the Established Hindsight Narrative has it, is supposed to be a band playing it safe.

So, what Fleetwood Mac set out to do on Tusk, at Buckingham’s behest, they had already begun unwittingly on Rumours. Tusk is Rumours with the vaseline wiped off the lens. It was meant to sound very different. It does sound very different, in parts; there’s nothing on Rumours that resembles Talking Heads (‘What Makes You Think You’re the One’) or Devo (‘Not That Funny’). In others parts, in Nicks’ scintillating feyness and McVie’s tender composure, it sounds very similar. But it isn’t very different, for all that they willed it to be. It makes manifest what Rumours lightly cloaked, turns the inside out. You can hear its bones rattle; but then, you can hear that on ‘Second Hand News’, which opens Rumours, too. On ‘The Ledge’ and ‘That's Enough For Me’, its dry, hectic rockabilly clatter prefigures The Birthday Party, whose debut album was a year away. While Tusk was completed around the time Bauhaus released ‘Bela Lugosi’s Dead’ - meaning neither of the two recordings could conceivably have influenced the other - there are similarities in the use of space, the persistent sense of foreboding.

I confess to mischief-making in my billing of this piece. I don’t really think Fleetwood Mac invented Goth, amusing as the idea is. Nor would I guess - although I don’t know - the protean Goths were listening intently, if at all, to Fleetwood Mac. Mind you, I bet plenty of them admired the style - visual and musical - of Stevie Nicks, who was surely an influence, whether directly or by osmosis, on the more hippyish and ethereal sides of Goth. Take her out of the equation and it is hard to imagine All About Eve, or the dreamier acts on the 4AD roster, having quite the same sound and aesthetic. It is not a long or twisting path between ‘Storms’ and ‘Martha’s Harbour’; while ‘Beautiful Child’ Goths up Country something lovely. But Goth was not just a sound, nor an aesthetic; it was (and is) as much as anything, a viewpoint, a way of seeing and feeling. Nicks excepted, Fleetwood Mac can have had very little do with that. Yet they did anticipate the sound of Goth, provided a premonition of it. I think Buckingham was drawing on the same sources to arrive independently at something that is at times eerily akin to it. Listen to the title track of Tusk: the ghostly backdrop of sound effects; the percussion, first claustrophobic then, thanks to the USC Trojan Marching Band, martial; the insinuating menace of its vocals, as if hissed and crooned from the recesses of a fever dream. You could drop ‘Tusk’ onto any number of moody, black-sleeved Eighties LPs, and while it would doubtless seem freakish - there isn’t an album in the world on which it wouldn’t, including its own - it would not impair the atmosphere one jot. It is dark, dark, dark. Again, you didn’t get that with Toto.

The 40th anniversary of Rumours coincides with the 30th of Tango in the Night, a record which has weathered very well indeed. While other big acts from the Seventies foundered in the Eighties, their attempts to keep pace with the times proving deadened and airless, Fleetwood Mac put a fashionable high gloss on their sound but kept their soul intact. ‘Big Love’, with its galloping percussion, its echo, its dramatic mood, and its gasps evocative of copulating robots, feels like a (perhaps accidental) companion piece to Kate Bush’s ‘Running Up That Hill’. ‘Everywhere’ is as lovely a thing as the undervalued McVie ever created. Which is saying something of the singer and writer who always provided the balm in a line-up that would otherwise have been wholly defined by Buckingham’s frenzies and anxieties, and Nicks’ otherworldly theatricality - amazing things in themselves, and made all the more appreciable by the contrast with McVie’s cool and soothing songs.

What Tango yet again demonstrated is that Tusk was not the anomaly it is now viewed as: rather, it was the plainest expression of something that was there from the moment Buckingham and Nicks joined the band. But for the extraordinary success of Rumours (45 million sold), Fleetwood Mac might have been understood as what they are: a solid blues-rock band who merged with a duo of disparate and phenomenal talents to create one of the greatest art-pop bands music has known. Then again, they might not have been: their principal sources (blues-rock, folk-pop, country-rock) are mistakenly seen as insufficiently synthetic for art-pop. But that’s what they were. The 12 years between Fleetwood Mac (1975) and Tango were a period when not only could the mainstream encompass such a thing, but when that thing could somehow find itself passed off as unchallenging. Rumours and Tusk are not just masterpieces. They are two sides of the same precious coin: one shiny, one scratched, both of them rare and beautiful.