When the 2005 Pirelli calendar is launched this week, you won’t be able to buy it in the shops, nor will you be able to send off for it. If you want one, your name has to be on a rather exclusive list. Only 400 copies of the annual pin-up bonanza are printed for the UK — and of those, one goes to the prime minister and six go to Buckingham Palace. The calendar is so sought-after that a 2003 edition signed by the photographer Bruce Weber recently sold on eBay for a staggering £9,000. Not bad for a girlie calendar produced as a marketing tool for an Italian tyre company.

The Cal, as it has become known, isn’t simply a glossy, upmarket porn fest for drooling motor-industry executives. From the start, Pirelli rewrote the rules of glamour photography. When the first edition appeared in 1964, London was the happening capital of the world, and fashion photographers, designers and models were the new arbiters of style. The calendar was a fusion of that talent, brought together by an enlightened client who didn’t interfere with their creativity.

Top names keenly awaited the commission. “You couldn’t be a photographer of international standing unless you had done the Pirelli calendar,” said Norman Parkinson, who shot the 1985 edition. And, indeed, the roll call of image-makers reads like a who’s who of contemporary photography: from Brian Duffy in the 1960s, through Terence Donovan, Herb Ritts, Annie Leibovitz and Richard Avedon, to Mario Testino and Nick Knight in the Noughties. But the calendar has also become a barometer of shifting fashion, taste and sexual attitudes — as much a sign of the times as the latest skirt length. In the 1964 edition, the only hint of sex was a bikini strap slipping off a model’s shoulder; by 1994, a totally naked Cindy Crawford was writhing on a sandy beach. And for this year’s calendar, with obvious sexiness out of fashion, Nick Knight tapped into the inner sexual world of the female rather than Pirelli’s more traditional male fantasies, with images based on conversations with 14 eminent women, from Tracey Emin to Catherine Deneuve.

At root, of course, the Cal is still about women’s bodies. “When Pirelli came along, it became okay to take off your clothes,” says the 1960s model Pat Booth.

“It made it respectable because of the photographers.” That’s the Pirelli line, too — even if Brian Duffy, who shot the 1965 edition, once dismissed the calendars as “tit’n’ass pictures”.

Cultural document or just a better class of porn? You decide.

The Pirelli Calendar — 40 Years Complete (Thames & Hudson £45) is out on November 22. The 2005 calendar, photographed by Patrick Demarchelier, is launched on Thursday

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