Second lines are stepping out of the shadows of main menswear collections. Once considered diluted versions of designer labels, they increasingly have their own strong identity and directional aesthetic. Until now, a certain element of snobbery has held back what used to be termed "diffusion lines" – collections rolled out by fashion designers offering customers labels at lower prices. Giorgio Armani instigated the concept in the early Eighties with Emporio Armani, prompting designers such as Dolce & Gabbana (D&G), Donna Karan (DKNY), Calvin Klein (CK) and Versace (Versus) to follow suit – and, unfortunately, in some cases dilute their brand identity.

Only a few notches up from the high street, these collections offered designer gear in simpler, more accessible forms. Regrettably, diffusion collections became associated with customers who couldn't afford the real McCoy, by offering watered-down designer wear to the masses. The ubiquitous logo-emblazoned T-shirt became the motif for diffusions' downfall.

After the turn of the millennium, the fashion industry became focused on exactly how to price its products, from the low end of fast fashion to the top realms of extreme luxury. As designer brands have recently clambered to retrieve their out-sourced licensing agreements and focus on the luxury end of the market, diffusion lines have been relegated to the back of the wardrobe.

Now, inspired by successful womenswear second lines, such as See By Chloé and Miu Miu (by Prada), designers are redefining these ranges. Whereas their top collections might focus on cut, fit and high-quality materials, second lines offer innovation in terms of fabric treatments, washes and prints, and enough of the cool factor to appeal to an avant-garde customer.

Cutting-edge Japanese designer Yohji Yamamoto recently announced the launch of Coming Soon for next winter, a second line created for customers who want clothing that does not advertise who designed it. Yamamoto describes the line as: "An act to express who you are through the essence of clothing, fabric and cut, for everyday comfort and style." The new line is the first major contemporary brand without the name of the designer emblazoned on the garment or label – a distinct change of direction from the early diffusion collections, which focused more on logo and less on good design.

"Ultimately, designers create second lines to appeal to a wider audience," says Anita Barr, the men's buying manager at Selfridges in London, whose second lines collections are enjoying a greater prominence in the store. They currently stock Raf By Raf Simons, Marc By Marc Jacobs, McQ (by Alexander McQueen) and K Karl Lagerfeld. "Certainly, when designers first introduced the idea of diffusion lines, they were made to appeal to a younger customer who couldn't necessarily afford the main line," Barr says. "But this seems to be less of a consideration now. Second lines provide an opportunity for a different type of creative outlet."

The success of Marc By Marc Jacob has set the standard for second lines by not sacrificing creativity in favour of cheaper prices. The label is regarded as a trend-setter, and inspires thousands of high street rip-offs each season. Tapping into aspirational customers' thirst for the Jacobs aesthetic, the Marc brand has its own identity and yet, importantly, still appeals to customers who buy the main collection.

"There is definitely a crossover between main line and second line customers," says Barr. "However, most successful second lines do have their own aesthetic and identity, and that's what customers really buy into." Some second lines aim to attract younger customers, or those who dress in a more relaxed way. "It's less to do with prices and more to do with attitude," Barr says. "We judge each collection that we have in-store on its own merit – how it will sit alongside our existing offerings and how our customers will react to it."

Alexander McQueen, who is backed by the Gucci group, launched his McQ second line in 2006; over 400 stores worldwide now stock it. "McQueen's main collection defines the spirit of Alexander McQueen, while McQ has a more renegade feeling," says the McQ brand manager, Jason Beckley. The second line is important for the McQueen business, as it brings the label's presence to a wider audience and complements the main collection.

Raf By Raf Simons launched in 2005. "I think about the second line very carefully," Simons says. "I have no interest in creating a brand that emblazons each T-shirt with one of my prints." Rather than be Raf Simons lite, the line aims to be a platform to explore the designer's favoured themes of youth culture, music and individuality. This leaves the main line free to focus on tailoring, experimental shapes and innovative materials. Simons knows the potential pitfalls of the second collection, however. "I don't want to be ruined, like other people have been by their second lines," he says.

Providing a core wardrobe of basics is where Karl Lagerfeld focused his second line, K Karl Lagerfeld, which launched this winter. "T-shirts and jeans are the base of modern dressing," Lagerfeld says. His denim-focused collection includes sweaters, trousers, blazers, trench coats and down-filled jackets, all in a functional palette of grey, black and white.

Clearly, for a second line to succeed, it must have a clear design identity while remaining sympathetic to its main collection. Second lines cannot make many compromises because of price, as they have to compete with high street heavyweights such as Topman, H&M and Zara. And second line ventures aren't without casualties. In 2006, Versace froze its Versus line; earlier this month, Prada discontinued its menswear Miu Miu line to focus on its more successful womenswear. Kim Jones's second line, KJ, launched last year, was shelved after only one season when the designer was hired to be Dunhill's creative director.

On a more positive note, the success of Marc By Marc Jacobs has encouraged the LVMH-backed label to consider a third, even cheaper collection, and Vivienne Westwood returned to the London catwalks after nine years to show her second line Red Label collection. It all suggests that designer firms, previously hooked on exclusive products in a booming luxury climate, are now reconsidering their positions to aspirational customers. And in doing so they are providing alternative wardrobe solutions and choice for men, which can only be a good thing.