Forty years after Bronx boy Calvin Klein went into business, his fashions have become legendary.

New York ? Forty years is a long time in the famously fickle fashion business, and Calvin Klein has remained a top-tier label through it all.

The company seems deserving of the big birthday party it’s throwing itself during New York Fashion Week, filling Manhattan’s new High Line park Sunday night with celebrities and other style stars.

But today’s Calvin Klein, owned by Phillips-Van Heusen Corp. with Francisco Costa and Italo Zucchelli as its creative forces, doesn’t look much like Calvin Klein’s Calvin Klein – the one the Bronx native opened with childhood friend Barry Schwartz back 1968. Back then, the duo made coats.

By the early ’70s, they expanded to women’s sportswear and eventually accessories, jeans, menswear, underwear, eyewear and cosmetics. In the process, Calvin Klein became a household name like few others in the fashion industry.

There have been waves of controversy, especially when it comes to the company’s signature sexy ads (think Brooke Shields or Kate Moss), as well as the chronicled ups and downs of Klein’s personal life, including rumors that he was dying in the ’80s and battling drugs in 2003.

The passing of the baton to hand-picked Costa and Zucchelli wasn’t seamless either, with both designers taking a few seasons to marry their aesthetics with the brand, and there have been business-side battles with licenses and funding.

Through all the changes, though, the design philosophy of Calvin Klein has been consistent, and that’s probably what has helped weather the rough patches. Calvin Klein, at its root, is chic and sophisticated. There’s boldness and strength from its sparseness.

“There’s something that’s intimidating about the brand that makes it aspirational,” says Costa. “The great thing about Calvin was there was always a subliminal, subtle message from the (runway) show that would set up a trend – it could be a hem, a lip color, a ponytail.”

Signature looks come in all black, all white, architectural dresses and sculptured coats. They’re luxurious without being showy. On the occasions garments are embellished, it’s done for effect – and because it’s done so rarely, it works.

Looking back at the fashions over four decades, it’s sometimes hard to tell one decade from another. Sure, there was some then-popular, now-dated gold lame in the disco years, and the uber low-waist jeans from the ’90s finally have fallen out of favor. But a sensual-yet-simple, body-conscious dress from early collections would look as stylish as ever today.

One of Costa’s proudest moments – save the two awards from the Council of Fashion Designers of America as the country’s best womenswear designer – came when he crafted a boiled wool coat for the current fall collection into essentially a piece of architecture without any artificial support. It took five tries to get it right.

“Things are not always easy, but if it was easy doing it, I’d be doing something wrong,” he says.

Costa has brought a more feminine silhouette to the collection, a youthfulness and a hint of color, but he sees himself as a steward, moving the brand forward without ever going off course.

“The idea is to create something new as much as we love to acknowledge the past,” he says. “Calvin’s legacy is here to stay, but it’s being adapted.”

New York Fashion Week, which kicks off Friday, will see some other milestones from designers who are fixtures on the runway scene:

Betsey Johnson: 30 years

Betsey Johnson’s styles have had many lives since Johnson and business partner Chantal Bacon started the label 30 years ago. The clothes have been worn by club kids, prom dates, rock stars and suburban moms, all enamored with Johnson’s flirty looks.

Johnson wasn’t new to the fashion scene when she started her own business: She worked at Paraphernalia, an influential store with the Warhol crowd, in the 1960s. It was there that Johnson learned to merge art and music into fashion – and that has been her trademark ever since.

The emergence of Lycra in the 1980s also became a big influence; it helped her make body-conscious clothes that tapped in to the interest of dance-inspired styles. Even at age 66, Johnson, a breast-cancer survivor, still wears those tight little numbers.

How else would she do the cartwheel at the end of each of her runway shows?

“It’s been a killer 30 years, and we can’t wait to see what the next 30 bring,” Johnson says in a statement for the AP.

DKNY: 20 years

DKNY is Donna Karan’s baby sister.

When Donna Karan, the woman and the designer who gave women their seven easy pieces, launched her namesake collection in 1985 she was targeting a career-minded customer much like herself. But a woman can only wear so many flattering bodysuits and draped dresses.

She launched DKNY to capture a more casual, youthful vibe, and, even as the label presents its 20th anniversary collection, that’s still the underlying mantra of the clothes.

The endless source of inspiration is Karan’s hometown of New York. Sometimes the vision comes from the creative community; the next season it’ll be the skyline.

Karan says the goal of DKNY is to be street smart, always with an urban mind-set. She calls DKNY “the pizza to the collection’s caviar.”

It’s a pizza with a lot of toppings, though: The label has expanded to include menswear, jeans, activewear, underwear and kids’ clothes.

Baby Phat: 10 years

Kimora Lee Simmons says she’s watched Baby Phat do a lot of growing up over the past decade – and not without growing pains. She wouldn’t have it any other way.

“I usually don’t go into businesses or endeavors that I don’t think will last. It’s like a child of mine, you expect it to grow, blossom and mature,” Simmons says during a recent telephone interview.

Baby Phat started as the women’s spinoff of her then-husband’s successful urban brand Phat Farm, but Baby Phat has become more design-conscious and, at times, sophisticated. It’s also a staple of Fashion Week, always drawing a celebrity-filled front row.

But, says Simmons, the high point of her 10 years in the fashion business comes each time she sees her cat logo on someone on the street. “It’s exciting to see your work in motion. Friends of mine have worn my clothes in concerts – I’ve seen Alicia Keys in my jeans – and I see my work in their work – that’s kind of fun. I don’t know if I’d say that if I was in business for 50 years, but it’s still a thrill to know people want it.”

Simmons sees a little uncertainty in the future as the company has been bought and sold and bought again, but she’s confident that the fashion industry is her calling. “We’re hoping the next 10 years will be more of the same, we hope it’s even better,” she says.