In a home studio in a garage in a suburb east of San Francisco, some time in 1999, Jerry Martin set about making the most inconspicuous music that he could. As the Audio Director at the video game developer Maxis, he'd already soundtracked detail-oriented simulation games like SimCity 3000 and SimCopter. But The Sims, the latest project handed down to him by the visionary game director Will Wright, would be different. The game would include all the endlessness of city-building, but none of the grandeur; it would demand the same patience as a life spent hovering over a city in a helicopter, but little of the action. Much of the playing time would be spent simply putting up walls, finding the right wallpaper, and arranging furniture. It wasn't really about anything in particular, and the music had to reflect that.

At the time, however, few people thought the game would catch on at all, despite the recent worldwide success of the SimCity series. Jeff Braun, the Maxis co-founder, has said that his company's board were dismayed by Wright's idea. "The board looked at The Sims and said, ‘What is this? He wants to do an interactive doll house? The guy is out of his mind,'" he recalled in a 2006 interview with The New Yorker. Martin, for his part, remembers it differently. He says the atmosphere at Maxis was broadly positive while EA, more heavily invested in sports and action games, didn't know what to do with a concept so strange as a life simulator.

It's likely one of the most widely heard album-length collections of music to be released so far this century. BusinessWire reported in 2005 that the original Sims had sold 16 million copies worldwide. Only four albums of original material released in 2000, the year The Sims hit shelves, have gone on to sell more copies: Linkin Park's Hybrid Theory, Britney Spears's Oops!... I Did It Again, Eminem's The Marshall Mathers LP, and The Backstreet Boys' Black & Blue. It's difficult to judge any record's true reach—radio and commercial play mean that those albums have had an impact beyond their sales figures. But Radiohead's Kid A, released that same year and long considered a seminal record, has sold around 1.5 million copies to date—roughly one-tenth of what The Sims has managed. It wouldn't be a stretch to say that more people have heard Jerry Martin's minimal jazz than Thom Yorke's measured howls on "Everything In Its Right Place."

But one section of the soundtrack to the original Sims has retained a unique power. The music for the game's Build Mode—composed by Martin alongside Doobie Brothers saxophonist Marc Russo and jazz pianist John R Burr—was made up of instrumental, new age, lone-piano pieces. The six songs eased into your headphones as soon as you paused time to do some ambitious landscaping or find the perfect roof. For a time in the early-2000s, millions of kids were listening to impressionistic, semi-improvised mood jazz without even knowing it.

Eighteen years on from its initial release, The Sims series is an institution. The four installments of the game (and the countless expansion packs that have followed) have been translated into 22 languages, selling around 200 million copies worldwide. And as the game has grown, the music has become slicker. My Chemical Romance, Flo Rida, and Kelly Rowland were among the artists who re-recorded songs in the game's unique Simlish language for The Sims 3__; the soundtrack to The Sims 4, released in 2014, was scored by the highly regarded British neoclassical composer Ilan Eshkeri.

"It was supposed to be very relaxing and very contemplative," Martin says over the phone from his new home in the mountains of central California. "You're sitting there and you're just kind of building and you're fiddling around with your house—that can just go on and on and on for hours. It's not like a big build-up to something."

Russo says that it turned out to be a "rigorous" process. Martin would give him a few words of direction—moods and vibes rather than tangible musical reference points—and Russo would come back to his neighbor with carefully arranged piano pieces. He'd walk across the street to Martin's studio, proudly record take after take on the keyboard, and then realize that Martin was only interested in a few bars of it. "I can remember many a time just pulling my hair out going, 'Man, I just wrote this great piece of music and he liked four measures of it that were in the middle,'" he recalls. "It worked out really well though. It made you think and go for things that you wouldn't normally go for."

With those radio stations eating up all of his genre-specific ideas—country, bossa nova, and classical music would all be present in the final game—Martin decided to work in some moody, genre-agnostic jazz. He invited Russo—who, in a situation that wouldn't seem out of place in The Sims, lived directly across the street—to contribute a couple of songs to the project.

Either way, as the project moved ahead, Martin was free to do as he pleased. Wright occasionally made suggestions, but those were usually for the radio stations that the players could turn on when their Sims were at home, dancing or relaxing. Build Mode (and Buy Mode, the soundtrack to which was significantly peppier ) was left to Martin's imagination.

When Russo and Martin couldn't quite capture that wistfulness themselves, they called Burr. A successful jazz pianist in his own right as well as a highly sought-after session musician, he was comfortable working off of Martin's ideas (and, unlike Russo, he was a pianist by trade rather than a saxophonist). Burr says he loved the cut-and-paste style of recording that Martin employed. It was a case, he says, of "just looking at Jerry and asking, 'Does this sound like what you want?'" So, while Russo went away and worked on his pieces only to see them chopped up by Martin, Burr was happy improvising. Though he's not listed as a songwriter on The Sims Soundtrack, the 16-track compilation that EA released in 2007, Burr's piano is there on just about every one of the Build Mode songs.

And unlike Martin, who had caught glimpses of the game in development and played around briefly with glitchy early demos, Russo had no feel for what would be happening on screen while his music played. "You were creating a palette without actually seeing it," he says. "Just purely taking on an emotion or a mood that has been sent your way, and then going with that." Those emotions and moods were difficult to pin down. Based on Martin's suggestions, Russo simply thought that there should be something "happy and innocent and hopeful" about the finished product.

The bigger challenge lay in the concept itself. After touring with The Doobie Brothers and playing in the Grammy-winning jazz group The Yellowjackets, Russo found it difficult to make intentionally unobtrusive music at first. "You would try to keep it so that a piece was interesting enough, but not so interesting that it took away from what was going on during a game," he says. "In my world, being a saxophone player and playing jazz or R&B or whatever it is, I'm there to make an impact, to set it up and make a splash."

Strangely, he is also the only person who can remember the music that inspired the compositions. He says that he's always admired the jazz-classical pianist Keith Jarrett, whose piano work shares a forlorn prettiness with the music that Burr worked on for The Sims soundtrack. George Winston, whose work wanders between folk and gentle impressionism, was in the back of Burr's mind at the time as well. "It was, for lack of a better term, that new age, Windham Hill sort of sensibility," he says.

It's good that he remembers all that, because Martin still doesn't know what to tell fans who ask about some next steps after binging the Build Mode songs on YouTube. "A lot of people ask, 'Where can I find that type of music?'" he says. "I don't really know! We did that because that's our style. We're into those styles. It was just a good combination of me writing it and [Burr] improvising on it."

All three musicians say that they have some sense of how far those inconspicuous songs have travelled now, though Burr certainly didn't grasp it immediately. He says that he visited his teenage nephew soon after the game was released and mentioned that he'd played some of the music for it. "I thought [Maxis] was some quaint, local video game company," he says. "So I thought this was a great coincidence. He said, 'this is the biggest game in the world.' That was news to me at the time."

Russo went to work for Maxis full-time for a year before returning to The Doobie Brothers, but he never got a chance to play The Sims. To this day, Martin remains the only one of the three who has messed around with the game, and even he says he's never had the time to really get to grips with it. Instead, he's been speaking to Burr about recording a follow-up to the Build Mode music—completely independent of The Sims, Maxis, and EA—and he's looking into the best way to crowdfund the project. For now, he's put the original tracks up on his website, and you can download them all for free.

For the millions of people who grew up semi-consciously absorbing these compositions, detaching them from the nostalgia they generate is impossible. The Sims might now be, as GQ's Lauren Larson wrote recently, better meditation than meditation itself—a low-stakes world of minutiae and minor wish fulfillment. But in the early 2000s, if you were still so young that you didn't know about cynicism, it allowed you a glimpse into a future that wasn't necessarily impossible. Maybe you could become a firefighter or a rockstar or a supermodel; maybe you could type in a cheat code and make a million dollars; maybe building a three-story mansion with an indoor swimming pool and a bunch of hot tubs and three wall-sized TVs isn't all that hard; maybe sadness is caused by hunger and hunger is cured by eating cereal and cereal is always in the kitchen and that's all there is to it.

Even though they hadn't played the game, the musicians who soundtracked its world were trying to capture some of that blissful naivety. Russo specifically remembers working one of the Build Mode songs—the fourth track, the one that was eventually renamed "If You Really See Eurydice." (The title is an esoteric reference to Greek mythology that Russo says he had nothing to do with.) It's something he still finds himself marvelling at now: "That's the really pretty one that when I do hear it I go, 'Oh my God, I came up with that—that's beautiful.'"

He says that he and Martin sketched out some ideas before he went away and worked on it in earnest. "I remember that that particular piece was just supposed to represent hope and a better world in the future," he says before trailing off into something he could only really sum up on a keyboard. "We'd throw around some catchphrases: hope, dream, with a hint of sadness because you're growing up, and you're leaving behind something that you really enjoy, and you're also looking ahead to a place where you hopefully will be filled with joy and wonder."