Prada's resort 2012 range foreshadows the pastels and rich hues, as well as the ultra-feminine silhouette, of the label's Fifties-esque spring show

'Pre-collections' used to be the preserve of women who wintered abroad. Now, the clothes that arrive between seasons have become one of fashion's biggest successes. Harriet Walker reports

As anyone who has been faced with racks and racks of brightly coloured T-shirts while it's hailing outside can attest, fashion seasons move in mysterious ways.

If you've ever tried to buy a coat only to be told that they sold out in August, pre-collections will doubtlessly be right up your street.

Like many things sartorial, they're actually nothing new: pre-collections – also known as resort or cruise – were originally for ladies who holidayed in winter and needed a wardrobe deemed 'out of season'. But in recent years the pre-collection has become something altogether more practical, not to mention commercially minded; its purpose now is to bridge the gap between a label's autumn and spring collections, which can, for restless retailers and skittish shoppers, often feel interminable, not to mention lagging behind when they have already been shown to the public six months previously.

Pre-collections add colour to the shopfloor during dreary months; they diversify from the standard seasonal variations; and, because they're a little more low-key, pre-collections can often be up to 20 per cent cheaper than their main collection equivalent. Buyers now report that pre-collections make up at least 50 per cent, if not more, of their orders per label per season.

"The season has grown in importance," wrote Bernadine Morris for The New York Times in 1989, "as European designers gain strength in this country and require new merchandise to fill their shops between fall and spring. In recent years, they have all added collections they call 'cruise' for American stores. They also find that these collections are gaining ground in Europe."

"In the past these pre-spring collections began arriving in November," says Ruth Runberg, buying director at Browns, "but many shipping dates have moved as early as 15 October. This is because brands realise that the earlier the goods are delivered, the longer selling window they have."

At this time of year, boutiques fill up with new things, which invigorate the rails ahead of Christmas and remain shiny until well into the new year, when tired autumn stock begins to be discounted. Pre-collection pieces rarely make it into the bargain bins, because they are less temporally oriented. These are quietly wearable clothes, which are not designed particularly for extremes of temperature so they don't get stale in the same way as the main collections can, arriving six months after they were first shown on the catwalks, only to remain in-store for another three.

Nor are they as conspicuous as the autumn and spring ranges, which define trends and make headlines, with certain pieces becoming a byword for certain designers, from Stella McCartney's polka dots to Céline's palazzo pants. Both set trends on the high street before the originals were even available to buy. Celebrities are frequently snapped on red carpets in pieces from collections that have only just been aired, wearing them a full half-year before anyone else can. Ubiquity becomes slightly inevitable and that, coupled with a rising awareness among savvy shoppers, is the kiss of death for any label.

"The catwalk show is a way for designers to really express themselves and create the mood for their collections and profile for their brand," explains Liberty's womenswear buyer Stephanie Jones. "The whole world has access to the show images – but in reality, they still need to create wearable fashion that real women will buy into. So pre-collections often aren't as exciting, but they're still very important."

"It's probably our biggest collection in terms of sales," says designer Jonathan Saunders. "It's more of a wardrobe suggestion as opposed to a silhouette suggestion or message for the season. It's like a core wardrobe that my customer wants and that's what's interesting about working on it. Often in collections, when you're trying to put a message forward, you see the same skirt in seven colours and the same silhouette."

If the pomp and ceremony of the autumn and spring collections is the time for showcasing concepts, skills and imagination, then the pre-collections are a chance to test-drive them first. Hence, the Balenciaga resort range by Nicholas Ghesquière – which is available now – prefigures the futuristic and boxy tailoring of the spring collection he showed in October and renders it less extreme (only slightly less, however). They're also a means of establishing a keynote for a label, of distilling its essence into a smaller capsule of pieces. As such, Phoebe Philo's pre-collection at Céline features the classic everyday staples for which she has become known: blouses, pleated skirts and modern, minimal trenchcoats. And it's also a chance for designers to work with new shades and see how they fit within an established repertoire – Prada's resort 2012 range foreshadows the pastels and rich hues, as well as the ultra-feminine silhouette, of the label's Fifties-esque spring show.

Pre-collections attract less publicity then the twice-yearly shows, but many labels do hold events for them. On the whole, New York is the venue for most – where fashion is more intensely commercial anyway – but Karl Lagerfeld showed his most recent resort collection for Chanel at the world's most expensive hotel, the Hotel du Cap in Antibes, having flown in models and much of the front row especially. The show featured classic Chanel bouclé suiting, spun in lemon yellow and lilac, as well as signature white tweed jackets to wear over black bandeau swimwear. "Resort" for some still means exotic holidays, after all.

"At Elle, we've been shooting pre-collection stories for around five years now," says acting editor Jenny Dickinson. "But in recent stories, we've placed more prominence on them. Pre-collections are beneficial all-round – the stores benefit from early deliveries; the designers benefit from getting a more commercial take on their catwalk offers; and the consumer benefits from something new and the designer's more accessible design."

In fact, pre-collections have become so intrinsic that even nascent labels and young designers are introducing them. London Fashion Week's Peter Pilotto launches its pre-collection this season, while up-and-coming designers are learning that they too must think about them and students are being inculcated at college.

"I think the growth of the internet in the past 10 years has led to a speeding up of the fashion system," says Andrew Groves, course director for fashion design at the University of Westminster, where students create a pre-collection during their final year. "We teach them to understand the key to a company's aesthetic signature and how to meet a customer's desires. They also need to understand the key pieces that make up a pre-collection, which can transcend seasons."

This has altered the mindset of the next generation of designers and shoppers, skewing them towards the pragmatic inter-season ranges rather than the more traditionally muscular main lines. Designer Tze Goh, 33, graduated from the acclaimed Central Saint Martins MA course last year.

"Since it is the buying and wearing of garments that really contribute to what fashion is, I would have to say that pre-collections are more important," he says. "I'm definitely considering and planning for pre-collections."

It's another example of how hard fashion has to work to stay relevant in a digital age of sell-sell-sell and nonexistent attention spans. The pre-collection may have been born out of mid-century millionaires needing to dress for Barbados in January, but it has turned into something inherently modern.