Sure, like everybody else, you’ve listened to Pink Floyd’s The Dark Side of the Moon so many times that you can recite not just every line but every heartbeat, clock tick, and cash register ring by heart. But how much do you really know about the landmark prog classic, which is celebrating its 40th anniversary this month?

To celebrate the 40 years we’ve been listening to what is arguably the preeminent rock album of the 1970s, here are 40 things you ought to know about Dark Side. Because lunacy breeds albums about lunacy, and albums about lunacy breed lunatic obsessions with album trivia. Let’s start with that iconic cover art, shall we?

The band members spent three minutes deciding on the front cover. Designer Storm Thorgerson brought seven designs into the Abbey Road studio where they were still recording. “The band trooped in, swept their gaze across the designs, looked at each other, nodded, and said ‘That one,’ pointing at the prism. Took all of three minutes,” Thorgerson recalled in liner notes for the 2011 deluxe box. In an 2003 interview, the designer elaborated, “No amount of cajoling would get them to consider any other contender, nor endure further explanation of the prism, or how exactly it might look. ‘That’s it,’ they said in unison, ‘we’ve got to get back to real work,’ and returned forthwith to the studio upstairs.”

One of the rejected designs involved a then-popular Marvel comic book superhero. Imagine how differently we’d think of the album if the Floyd members had expressed any interest in one of Thorgersen’s alternative ideas, to have the cover feature… the Silver Surfer!

The band had always hated having their photos in the artwork. “When Storm showed us all the ideas, with that one, there was no doubt,” guitarist David Gilmour told Rolling Stone in 2003. “It was, ‘That is it.’ It's a brilliant cover. One can look at it after that first moment of brilliance and think, ‘Well, it's a very commercial idea: It's very stark and simple; it'll look great in shop windows.’ It wasn't a vague picture of four lads bouncing in the countryside. That fact wasn't lost on us.”

It was keyboardist Rick Wright who was insistent that the cover not feature any photography at all, even conceptual photos. The Hipgnosis design team was famous for elaborately staged and photographed covers, like Wish You Were Here, which came out two years later. But in this instance, as Thorgerson remembers it, Wright “said, ‘Storm, let’s have a cool graphic, not one of your tatty [figurative] pictures…’ I protested. ‘Rick,’ I said, ‘I do images, I don’t do cool graphics.’… Whereupon Rick said, ‘Why don’t you try to see it as a challenge.’”

The prism design was partly inspired by Floyd’s extravagant live light shows. “The refracting glass prism referred to Floyd light shows–consummate use of light in the concert setting,” Thorgerson said in an interview for the album’s 30th anniversary. “Its outline is triangular and triangles are symbols of ambition, and are redolent of pyramids, both cosmic and mad in equal measure, all these ideas touching on themes in the lyrics. The joining of the spectrum extending round the back cover and across the gatefold inside was seamless like the segueing tracks on the album, whilst the opening heartbeat was represented by a repeating blip in one of the colors.”

The designer went to Egypt to shoot infra-red photography of the pyramids for an inside poster. Pyramids are triangular, like the prism on the front cover, so there was that angle. But Thorgerson also figured pyramids tied in with the album’s running theme of insanity, being “fantastic structures intended to elevate Pharaohs and assist in transporting worldly goods skywards to heaven—and how mad is that?”

For a while the album had a different working title. It was to be named Eclipse (A Piece for Assorted Lunatics].

The reason it had a different title for a while was because there’d just been another album come out called Dark Side of the Moon. A group called Medicine Head beat them to the punch with a 1972 release by that title, which made Pink Floyd temporarily drop it as theirs. But when the Medicine Head album flopped, the original title was a go again.

“Money” is one of the few hit singles ever to utilize a 7/4 time signature. Roger Waters has made it sound like David Gilmour wasn’t down with that weird rhythm. “Occasionally,” Waters told Rolling Stone, “I would do things and Dave would say, ‘No, that's wrong. There should be another beat. That's only seven.’ I'd say, ‘Well, that's how it is.’ A number of my songs have bars of odd length.” But part of the song does take place in a traditional time signature. As Gilmour said, "We created a 4/4 progression for the guitar solo (but) made the poor sax player play in 7/4."

“Money” was influenced by… Booker T and the MGs? Though the basis of the song is a blues progression written by Waters, Gilmour has said he brought an R&B influence to the song’s instrumental breaks. “I was a big Booker T fan,” said Gilmour. “I had the Green Onions album when I was a teenager. And in my previous band… we played ‘Green Onions’ onstage… It was something I thought we could incorporate into our sound without anyone spotting where the influence had come from. And to me, it worked. Nice white English architecture students getting funky is a bit of an odd thought.”

The opening “song,” “Speak to Me,” is credited solely to drummer Nick Mason, something Waters has insisted was an act of charity. "God, I resent giving that to him now,” Waters said. “'Cause he had nothing to do with it... It was like a gift. It was all right at the time."

“Us and Them” was originally written and submitted three years earlier for the soundtrack of the film Zabriskie Point. Antonioni’s loss was Dark Side’s gain. "We wanted to put it on Zabriskie Point, on the sequence where they're having the riots and the police beating heads on UCLA campus; the counterpoint between that slow, rather beautiful music and this violence going on was great,” said Gilmour. "We couldn't understand it when Antonioni said: 'Ees not quiiite riiight for thees beet'.”

“Breathe” emerged from a song of the same name that Waters wrote for the soundtrack of a documentary called The Body, also three years earlier. But Waters’ two “Breathe” songs ultimately don’t share much besides a title and an opening line.

At the very end of “Eclipse,” in the right channel, there is the faint sound of a Muzak version of the Beatles’ “Ticket to Ride.” This was apparently playing in the background when they recorded the closing snippet of chatter. So far as we know, the Fab Four never demanded any royalties over this.

Session vocalist Clare Torry did demand—and receive—royalties and co-writing credit for “The Great Gig in the Sky,” more than 30 years after the fact. She prevailed in court, although terms of the settlement were not released; all we know is that she has been credited as a writer on all reissues since 2005. Originally she’d only been paid 30 quid for her brief studio session, and that was double the going rate because she’d come in on a Sunday night. But over the years she’d come to believe that her contributions amounted to a melody that had not previously existed in Rick Wright’s instrumental composition. Asked why she’d waited so long to file suit, she told writer John Harris, “Over the years, people said to me on numerous occasions, ‘What are you going to do?’ I did look into it, and at first, the costs were prohibitively expensive... And also, if I’d started something when I was well into my career, I’d have been thought of as a troublemaker. So once I’d retired, I thought about it again. It went on from there.”

Contrary to everything you might have assumed from the audio alone for 40 years, Clare Torry is a white chick. “We’d been thinking Madeleine Bell or Doris Troy and we couldn’t believe it when this housewifely white woman walked in,” Gilmour told Mojo. “But when she opened her mouth, well, she wasn’t too quick at finessing what we wanted, but out came that orgasmic sound we know and love."

“The Great Gig in the Sky” originally had found bits of religious voice-over instead of a female vocal. Before the band recorded it, they played it live over a period of a year as an instrumental. It was then known as “The Mortality Sequence” and used taped snippets of a voice reading from the biblical Book of Ephesians and a BBC talk show hosted by theologian Malcolm Muggeridge.

Torry’s main direction: sing for several minutes, and don’t sing any words. Waters recalled, “Clare came into the studio one day, and we said, ‘There's no lyrics. It's about dying—have a bit of a sing on that, girl.’ I think she only did one take. And we all said, ‘Wow, that's that done. Here's your 60 quid.’” (By Torry’s recollection, it was two and a half takes, and 30 quid.)

Torry thought the session went disastrously and was sure they had ditched her vocal until she saw the album in a King’s Road record shop. “I went in, put the headphones on, and started going ‘Ooh-aah, baby, baby - yeah, yeah, yeah.’ They said, ‘No, no – we don’t want that. If we wanted that we’d have got Doris Troy.’ They said, ‘Try some longer notes’… I remember thinking to myself, ‘I really, really do not know what to do. And perhaps it would be better if I said “Thank you very much” and gave up.’ “ But rather than give up and exit the session, “That was when I thought, ‘Maybe I should just pretend I’m an instrument’.” The second take was outstanding, she recalls, but she said she stopped the third take because “it was beginning to sound contrived. I said, ‘I think you’ve got enough.’ I thought it sounded like caterwauling.” She left Abbey Road thinking it was an experiment that hadn’t worked out. “I honestly thought it would never see the light of day” until she went by a record store and saw her name was indeed in the credits.

It was engineer Alan Parsons’ idea to bring Torry in for the “Gig” vocal. Parsons claims he heard her singing a cover of “Light My Fire”—although she disputes that, claiming she never sang the Doors’ song in her life. At the time she got the call, the only Pink Floyd song she knew was “See Emily Play,” “and that didn’t really hit the spot with me,” she said. “They weren’t my favorite band. If it had been the Kinks, I’d have been over the moon.” No pun intended, we're sure.

Parsons credits the band’s addiction to Monty Python’s Flying Circus for his ability to work in his ideas while they were distracted. “Very often, they'd stop for Monty Python and leave me to do a rough mix,” Parsons told Rolling Stone. “That was quite fulfilling for me. I got to put my own mark on it.”

There has been some acrimony between Alan Parsons and the Floyd members over the decades. “I think they all felt that I managed to hang the rest of my career on Dark Side of the Moon, which has an element of truth to it,” said Parsons, who went on to have hits with the Alan Parsons Project. But, he told Rolling Stone, “I still wake up occasionally, frustrated about the fact that they made untold millions and a lot of the people involved in the record didn't.” In 2011, when Dark Side was turned into a boxed set, Parsons’ quadrophonic mix was rejected in favor of a new one, and he was not invited to participate in any way. “They’ve seen it fit not to give me [a copy] yet,” he said. “That’s very typical of the situation over the last 40 years or so. On many occasions I’ve asked to be recognized for my contributions to The Dark Side of the Moon, but both the band and the label have declined to give any sort of gesture towards me.”

The entire song cycle had been illegally issued as a bootleg LP long before the band finished the album. The group first attempted to perform Dark Side in its entirety live at a January 1972 concert, but a tape machine broke down, so they had to cut the performance of the piece off after “Money.” After that there were fewer technical snafus and the work-in-progress was performed throughout 1972, even though some of the studio work took place in January 1973, less than two months before the album was released. A performance of the entire piece at England’s Rainbow Theatre in February 1972 became a popular vinyl bootleg that year.

Future super-producer Chris Thomas was brought in for the mixing process—possibly to be a mediator between Gilmour and Waters. Recalled Gilmour in a 1993 interview with Guitar World: “Chris Thomas came in for the mixes, and his role was essentially to stop the arguments between me and Roger about how it should be mixed. I wanted Dark Side to be big and swampy and wet, with reverbs and things like that. And Roger was very keen on it being a very dry album. I think he was influenced a lot by John Lennon's [Plastic Ono Band], which was very dry... We were going to leave Chris to mix it on his own, with Alan Parsons engineering. And of course on the first day I found out that Roger sneaked in there. So the second day I sneaked in there. And from then on, we both sat right at Chris' shoulder, interfering. Luckily, Chris was more sympathetic to my point of view than he was to Roger's.”

Waters seems to think he won that battle for mixer Chris Thomas’ affections. Waters singled out Thomas’ contribution as invaluable when I interviewed him for the 20th anniversary of the album in 1993. “I think people who feel it was brilliantly made, what they're noticing is the fact that when there's something important happening, whether it's a cash register or a lead vocal or a guitar solo or footsteps in a tunnel, you can hear it,” Waters told me then. “There is space around it. And I think that that's partially at least a function of the quiet drums, which is a nice thing about it, and partially a function of Chris Thomas, who mixed it and gave the lead stuff space to exist in.”

Actress Naomi Watts’ father makes a cameo, as the “stoned laugher.” The unbridled cackling that prominently pops up in two of the tracks is the voice of Peter Watts, who was the group’s road manager. He’s also seen on the back cover of Ummagumma. He died of a drug OD in 1976, when Naomi was only 8.

Wings member Henry McCullough is also one of the uncredited voices—but bosses Paul and Linda didn’t make the cut. That’s McCullough at the end of “Money” saying “I don’t know; I was really drunk at the time,” supposedly talking about a fight he’d had with his wife. Roger Waters “interviewed” people who were hanging around Abbey Road to get the snippets of dialogue on the album, and although the McCartneys took part in these recordings, none of their bits were used because it was thought they were trying too hard to be funny.

Dark Side still holds the record for the most number of weeks spent on the Billboard top 200. The album spent a record 591 consecutive weeks on the Billboard Top 200 chart between 1976 and 1988. Counting the time it spent there before and since, it totaled out at 741 weeks, beating the former record-holder, Johnny's Greatest Hits, by Johnny Mathis, by several years. (Eventually Billboard changed the rules so that albums automatically reverted to the catalog chart and not the current chart if they dropped off for a period of time.)

It is certified for sales of 15 million copies in America, although it has almost certainly sold many, many millions more than that. The 15-times-platinum certification came all the way back in 1998. Why the album has not been submitted for further certifications since then is a mystery. We do know that SoundScan has it down as selling over 9 million copies since that system was instituted in 1991, at a time when Dark Side was RIAA-certified for 12 million, so, doing the math, we could guess that it has sold about 21 million in the U.S.

Waters says he was under pressure to let Gilmour do most or all of the singing on Dark Side. “"My memory is David and Rick were at great pains to point out how I couldn't sing and how I was tone-deaf," he told Rolling Stone in 2011. "And there's this bollocks that Rick had to tune my bass. And you only have to look at the body of work to realize that this is not the case. Maybe their way of keeping me from being totally overwhelming was to point out that I might have vocal and instrumental inadequacies."

The version of Floyd fronted by Waters and Gilmour ceased to exist after Waters announced his intention to exit in 1985. Waters was outraged—and filed suit—when the others soon decided to carry on without him. He told Rolling Stone in 1987, "There was no point in Gilmour, Mason or Wright trying to write lyrics. Because they'll never be as good as mine. Gilmour's lyrics are very third-rate. They always will be. And in comparison with what I do, I'm sure he'd agree."

After some years had passed since the split, Roger Waters and the new lineup of Pink Floyd took to performing The Dark Side of the Moon in its entirety—separately. In 1994, the Waters-less Floyd performed Dark Side on tour, including at a 14-night stand in London, where they invited Waters to join them, in vain. “"I thought it would be a good thing for the fans," Gilmour said. "But also with the safety cushion of knowing that he wouldn't do it. It was a genuine offer, though.” In 2006, Waters played on the album in its entirety on tour joined by drummer Nick Mason; he had a Gilmour sound-alike sing the estranged guitarist’s parts. Meanwhile, Gilmour, who had long since retired the Floyd name, took Wright out with him for a solo tour that included Dark Side selections.

In 2005, the classic lineup reunited for one mini-set at Live 8. This one-time gig included three songs from Dark Side (“Speak to Me,” “Breathe,” and “Money”) as well as two later choices (“Comfortably Numb,” “Wish You Were Here”).

Neither of the band’s two frontmen consider Dark Side their best album. Waters told me in 1993 that The Wall is "a much more important work." Gilmour has a different pick. “For me, Wish You Were Here is the most satisfying album,” he told Guitar World. “I'd rather listen to that than Dark Side of the Moon. Because I think we achieved a better balance of music and lyrics on Wish You Were Here. Dark Side went a bit too far the other way—too much into the importance of the lyrics. And sometimes the tunes—the vehicles for the lyrics—got neglected. To me, one of Roger's failings is that sometimes, in his effort to get the words across, he uses a less-than-perfect vehicle.”

Waters has long been frustrated with what he sees as Gilmour’s who-cares-about-the-lyrics stance. When I interviewed Waters in 1993 for the Los Angeles Times about the album’s 20th anniversary, he said pointedly: “If you read the interviews with my ex-colleagues, you'll find that they all say, 'We weren't really very interested in the words or what the record was about.'" He hated the idea that people would put on the album for pure ambience. For him, it was meant to be cerebral, and listened to while in one's right mind. But "through the intervening years, I've very much picked up a feeling that Dark Side of the Moon is easy-listening, wafting sort of music: You turn the lights down low and... drift away into some sort of New Age blissful state. And that's always confused me, because I at the time had thought the songs were actually about something more than that."

The album has had numerous full-length cover versions. Phish and Dream Theater are among the bands who’ve covered the album in its entirety in concert, and the Flaming Lips released a studio version of their take on the album. There have been bluegrass, a cappella, and string-quartet album versions, not to mention the notorious, reggae-died Dub Side of the Moon.

“On the Run” is often used by the Chicago Bulls as background music when opposing teams are introduced. Whether that’s because the tape loop just sounds cool or because they like to subliminally remind other teams that they plan to keep them on the run isn’t altogether clear.

Before Dark Side, Pink Floyd considered and dismissed another concept album, Household Objects. This LP would have consisted of the band playing, yes, household objects instead of instruments. One artifact from that, an instrumental called “The Hard Way,” was included on the 2011 Dark Side boxed set. Although it’s a fascinating what-if, we can all be grateful they decided guitars, synths, and madness ‘n’ mortality were the way to go with the follow-up to Meddle after all.

The album won one whole Grammy award. That went to Parsons, for best-engineered recording.

The band members swear they never watched or even thought about The Wizard of Oz while making the album. But that hasn’t stopped tens of millions of conspiracy theory-loving dreamers from synching the two up. Go for it!