League of gentlemen: How to get the effortlessly cool style of Paul Newman and Steve McQueen

Graham Marsh is waxing lyrical about his loafers. He removes one from his (sockless) foot and places it on the table in front of him. "They're made of horse leather," he says, bending it back and forth. "They use only a certain cut from the hind quarter of the back leg so it's slightly thicker than normal leather, which means you get softer creases across the front." The shoe in question was produced by an American company called Alden, founded in New England more than 100 years ago. "They make the best footwear," says Marsh, "They're spot on. The cat's whiskers."

His friend JP Gaul sits beside him nodding in agreement, then removes his loafer from his (also sockless) foot. "These are Scotch grain penny loafers," he says, "They're about 25 years old. The company that made them doesn't exist any more; they just get better and better with age."

It's a rare thing to hear two grown men deliberating over creases in shoes and other such pressing sartorial issues in the way that Marsh and Gaul do. They speak lovingly about turn-ups on jeans, buttons on shirts and the dimensions of belt loops. The focus of their obsession is one very specific period in American clothing history: the era between the ' late 1950s and early 1960s when a new period of post-war prosperity brought about a revolution in fashion. They have just produced a book entitled The Ivy Look, an homage to this dress code, which Marsh admits has become an addiction.

For those unfamiliar with the look, it started on the campuses of the Ivy League universities but quickly filtered down to become the dominant American male style of the time. Everyone from President Kennedy to Woody Allen, and John Coltrane to the ordinary working man, wore it. It was personified in the heritage and craftsmanship of iconic Ivy brands such as J Press, Chipp and, with its legendary flagship store on Madison Avenue, Brooks Brothers. "It was all about looking modern and pared down," says Marsh. "There were no frills and no extras, just clean lines with natural shoulders. It was about being comfortable but looking smart at the same time. It's a really, really hard look to perfect."

In the book there are numerous examples of how the Ivy look is best done. It's Miles Davis in a green Oxford cloth button-down shirt on the cover of his 1958 album Milestones. It's Steve McQueen in a pair of brown-suede crepe-soled boots in the 1968 film Bullitt. And it's Paul Newman in 1956, displaying effortless Ivy style simply by virtue of his khaki trouser/corduroy jacket combination.

To the untrained eye, the Ivy look is not easy to spot, as it's all about detail. The seminal Ivy shirt, for example, would be one with a back button, centre pleat, back loop and a six-button placket so the roll on the button-down collar sits just so. For Marsh, it's also 1947 vintage Levi's (of which he's got about 15 pairs), a Baracuta Harrington jacket and his beloved Alden or Bass Weejun loafers.

"It's the little things," says Gaul. "When you fall in love with these clothes you fall in love with certain small details and they become fixed in your head. If you've got the eye, and I would claim we have the eye, you can see the difference."

Marsh, now 65, discovered the Ivy look when he was working at Marvel comics in London in the early 1960s. He used to frequent a club that ran at lunchtimes at the Lyceum to get his fix of the music of Miles Davis, Gerry Mulligan and Jimmy Smith. "The whole connection between music and the Ivy look was really important. Modern jazz, the old Blue Note album covers, post-war graphic design – it was all mixed in. It was a time when we were obsessed with anything modern."

Marsh hung around with the equally clothes-obsessed original London mods, a group who heavily drew on the Ivy style but also looked to Italy for their influences. "I was never a mod, I was always an Ivy Leaguer; for me it was always about America," says Marsh. "Plus I never drove a scooter as I didn't want to mess up my clothes." Back then, they had no qualms about judging people by what they wore. First they'd look at their shoes, then at the cut of the rest of the wardrobe. "If you weren't wearing what was considered to be an acceptable shoe," says Marsh, "then you were met with frosty indifference."

Gaul, who is 20 years younger than Marsh, grew up in Liverpool wishing he had been born two decades earlier. "I was a teenager in the 1980s and just hated it," he says. "I hated the music. I hated that whole Echo and the Bunnymen granddad- coat look. It left me really cold." By now, the Americana that Marsh had fallen in love with had very much changed too. "America at that time was Ronald Reagan and dodgy right-wing politics, it wasn't a particularly glamorous world."

Gaul found his way into the Ivy world after becoming a mod and moving to London. He got a job working in the legendary J Simons clothes shop in Covent Garden, which was where he met Marsh. John Simons also had another store, The Ivy Shop in Richmond, and between them the three became the fulcrum of the whole scene. "John was my guru," says Gaul. "I had to learn the look. It was like a code and it had to be explained to me."

In many ways, however, since the 1980s at least, Ivy League has had something of a bad press. It was around this time that big brands picked up on the look. Ralph Lauren (who incidentally started out as a salesman in Brooks Brothers) marketed it so successfully that, on the high street at least, some believe his name is synonymous with true Ivy style. Giorgio Armani produced a slightly more subtle version – taking the soft Ivy silhouette, with its natural shoulder shape, and producing a range of slouchy, elegant suits. ("Think Richard Gere in American Gigolo," says Gaul.) Even so, by the end of the decade, Ivy style was often confused with preppy, waspish or just plain old posh. In mid 1996, the Ivy Shop in Richmond shut down, and in February this year, faced with spiralling business costs, J Simons finally closed its doors too.

"In the 1980s, associations with Ivy changed and it went from that 1960s thing to becoming associated with dodgy posh boys," says Gaul. "That collar-up-on-the-rugby-shirt look that you see on rich kids in Parsons Green is what some people mistake it for, but it's not that. With Ivy it's all in the detail and the balance, so when it's done wrong, it's done really wrong. What I love about Ivy, and what drew me to it in the first place, is that it's a totally democratic, classless dress code. It's about working-class kids wanting to look a bit sharp and wearing decent, affordable clothing with style and heritage."

Recently, however, there are signs that things are moving in the right direction again. The hit TV series Mad Men – with Don Draper in his slim-fitting suits and narrow ties courtesy of Brooks Brothers the embodiment of Ivy style – has obviously had a massive influence. And "recently," says Marsh, "both Prada and Dior have done some very Ivy-influenced suits and trousers." In 2008, meanwhile, the preppy British high-street brand Jack Wills launched a spin-off called Aubin and Wills, which has its flagship in London's Shoreditch and peddles a more mature range much closer to the true Ivy look. "It's a style you see more and more today," says Gaul. "It's kids in their twenties wearing narrow chinos really short with no socks. It's the revival of the boat shoe and it's wearing Wayfarer glasses. Whether they know it's American or not, I don't know, but that's where it directly comes from."

And, of course, the internet has helped. There is plenty of trading on eBay, and a fine selection of classic Ivy League imagery on blogs such as ivy-style.com. Gaul tells me how he has just discovered a website set up by one Ivy fan in Chicago. "He offers a service to afflicted Ivy League obsessives where he goes round all the thrift shops, lists all the good stuff they've got and sells it on for peanuts." Gaul is currently awaiting his first order of a pair of Florsheim wing-tip brogues and two more pairs of Bass Weejun loafers to add to his already extensive collection.

But more exciting to Ivy Leaguers is recent news that John Simons is opening a new store in central London later this year, which he promises will offer the best Ivy collection he's done. Things, it seems, are finally looking up for the Ivy League. "I think it's a look that's never been improved upon," concludes Gaul. "Things have happened along the way, but it still looks fresh, it still pulls people in, whether they know where it comes from or not. Part of its magic is that it is so low key. It's like a code or some kind of shorthand. You know when someone understands it and that's enormously satisfying to see."

'The Ivy Look' by Graham Marsh and JP Gaul (Frances Lincoln, £12.99) is out now in paperback